cephalic mass there is now formed a large cavity (Fig. 223, d), by the melting down and liquefaction of a portion of its substance. This cavity is the pharynx. It corresponds by its anterior extremity to the future situation of the mouth; and by its posterior portion to the upper end of the intestinal canal, the future situation of the stomach. It is still, however, closed on all sides, and does not as yet communicate either with the exterior or with the cavity of the stomach. There is, accordingly, at this time, no thorax whatever; but the stomach lies at the upper extremity of the abdomen, immediately beneath the lower extremity of the pharynx, from which it is separated by a wall of intervening tissue.

Subsequently, a perforation takes place between the adjacent extremities of the pharynx and stomach, by a short narrow tube, the situation of which is marked by the dotted lines x, in Fig. 223. This tube afterward lengthens by the rapid growth of that portion of the body in which it is contained, and becomes the œsophagus. Neither the pharynx nor oesophagus, therefore, are, properly speaking, parts of the intestinal canal, formed from the internal layer of the blastodermic membrane; but are, on the contrary, formations of the external layer, from which the entire cephalic mass is produced. The lining membrane of the pharynx and oesophagus is to be regarded, also, for the same reason, as rather a continuation of the integument than of the intestinal mucous membrane; and even in the adult, the thick, whitish, and opaque pavement epithelium of the oesophagus may be seen to terminate abruptly, by a well-defined line of demarcation, at the cardiac orifice of the stomach; beyond which, throughout the remainder of the alimentary canal, the epithelium is of the columnar variety, and easily distinguishable by its soft, ruddy, and transparent appearance.

As the oesophagus lengthens, the lungs are developed on each side of it by a protrusion from the pharynx, which extends and becomes repeatedly subdivided, forming the bronchial tubes and their ramifications. At first, the lungs project into the upper part of the abdominal cavity; for there is still no distinction between the chest and abdomen. Afterward, a horizontal partition begins to form on each side, at the level of the base of the lungs, which gradually closes together at a central point, so as to form the diaphragm, and finally to shut off altogether the cavity of the chest from that of the abdomen. Before the closure of the diaphragm, thus formed, is complete, a circular opening exists on each side the median line, by which the peritoneal and pleural

cavities communicate with each other. In some instances the development of the diaphragm is arrested at this point, either on one side or the other, and the opening accordingly remains permanent. The abdominal organs then partially protrude into the cavity of the chest on that side, forming congenital diaphragmatic hernia. The lung on the affected side also usually remains in a state of imperfect development. Diaphragmatic hernia of this character is more frequently found upon the left side than upon the right. It may sometimes continue until adult life without causing any serious inconvenience.

The heart is formed, at a very early period, directly in front of the situation of the oesophagus. Its size soon becomes very large in proportion to the rest of the body; so that it protrudes beyond the level of the thoracic parietes, covered only by the pericardium. Subsequently, the walls of the thorax, becoming more rapidly developed, grow over it and inclose it. In certain instances, however, they fail to do so, and the heart then remains partially or completely uncovered, in front of the chest, presenting the condition known as ectopia cordis. This malformation is necessarily fatal.

Development of the Face.-While the lower extremity of the pharynx communicates with the cavity of the stomach, as above described, its upper extremity also becomes perforated in a similar manner, and establishes a communication with the exterior. This perforation is at first wide and gaping. It afterward becomes divided into the mouth and nasal passages; and the different parts of the face are formed round it in the following manner :

Fig. 224.

From the sides of the cephalic mass five buds or processes shoot out, and grow toward each other, so as to approach the centre of the oral orifice above mentioned. (Fig. 224.) One of them grows directly downward from the frontal region (1), and is called the frontal or intermaxillary process, because it afterward contains in its lower extremity the intermaxillary bones, in which the incisor teeth of the upper jaw are inserted. The next process (2) originates from the side Longet; from a specimen in the of the opening, and, advancing toward the median line, forms, with its fellow of the

collection of M. Coste.-1. Frontal or intermaxillary process. 2. Process of superior maxilla. 3. Process of inferior maxilla.

opposite side, the superior maxilla. The processes of the remaining pair (3) also grow from the side, and form, by their subsequent union upon the median line, the inferior maxilla. The inferior maxillary bone is finally consolidated, in man, into a single piece, but remains permanently divided, in the lower animals, by a suture upon the median line.

Fig. 225.

HEAD OF HUMAN EMBRYO at the end of the first month. After Longet; from a specimen in the collection of M. Coste 1. Ala uasi. 2. Superior maxilla.

As the frontal process grows from above downward, it becomes double at its lower extremity (Fig. 225), and at the same time two offshoots show themselves upon its sides (1), which curl round and inclose two circular orifices (s), the opening of the anterior nares; the offshoots themselves becoming the alæ nasi.

The processes of the superior maxilla continue their growth, but less rapidly than those of the inferior; so that the two sides of the lower jaw are already consolidated with each other, while those of the upper jaw are still separate.

As the processes of the superior maxilla continue to enlarge, they also tend to unite with each other on the median line, but are

3. Inferior maxilla. 4. Inter- prevented from doing so by the intermax

maxillary process.

6. Eye.

5. Nostril.

illary processes which grow down between them. They then unite with the intermaxillary processes, which have at the same time united with each other, and the upper jaw and lip are thus completed. (Fig. 226.)

Fig. 226.

The external edge of the ala nasi also adheres to the superior maxillary process and unites with it, leaving only a curved crease or furrow, as a sort of cicatrix, to mark the line of union between them.

Sometimes the superior maxillary and the intermaxillary processes fail to unite with each other; and we then have the malformation known as hareThe fissure of hare-lip, conse

lip.

HEAD OF HUMAN EMBRYO, about quently, is never exactly in the median

the end of the second month -From a

specimen in the author's possession.

line, but a little to one side of it, on

the external edge of the intermaxillary process. Occasionally, the same deficiency exists on both sides, producing "double hare-lip;" in which case, if the fissures extend through the bony structures, the central piece of the superior maxilla, which is detached from the remainder, contains the four upper incisor teeth, and corresponds with the intermaxillary bone of the lower animals.

The eyes at an early period are situated upon the sides of the head, so that they cannot be seen in an anterior view. (Fig. 224.) As development proceeds, they come to be situated farther forward (Fig. 225), their axes being divergent and directed obliquely forward and outward. At a later period still they are placed on the anterior plane of the face (Fig. 226) and have their axes nearly parallel and looking directly forward. This change in the situation of the eyes is effected by the more rapid growth of the pos terior and lateral parts of the head, which enlarge in such a manner as to alter the relative position of the parts seated in front of them.

The palate is formed by a septum between the mouth and nares, which arises on each side as a horizontal plate or offshoot from the superior maxilla. These two plates afterward unite with each other upon the median line, forming a complete partition between the oral and nasal cavities. The right and left nasal passages are also separated from each other by a vertical plate (vomer), which grows from above downward and fuses with the palatal plates below. Fissure of the palate is caused by a deficiency, more or less complete, of one of the horizontal maxillary plates. It is accordingly situated a little to one side of the median line, and is frequently associated with hare-lip and fissure of the upper jaw. The fissures of the palate and the lip are very often continuous with each other.

The anterior and posterior pillars of the fauces are incomplete vertical partitions, which grow from the sides of the oral cavity, and tend to separate, by a slight constriction, the cavity of the mouth from that of the pharynx.

When all the above changes are accomplished, the pharynx, œsophagus, mouth, nares, and fauces, with their various projections and divisions, have been successively formed; and the development of the upper part of the alimentary canal is then complete.

CHAPTER XVI.

DEVELOPMENT OF THE KIDNEYS, WOLFFIAN BODIES, AND INTERNAL ORGANS OF GENERATION.

THE first trace of a urinary apparatus in the embryo, consists of two long, fusiform bodies, which make their appearance in the abdomen at a very early period, situated on each side the spinal column. These are known by the name of the Wolffian bodies. They are fully formed, in the human subject, toward the end of the first month (Coste), at which time they are the largest organs in the cavity of the abdomen, extending from just below the heart, nearly to the posterior extremity of the body. In the foetal pig, when a little over half an inch in length (Fig. 227), the Wolffian bodies are rounded and kidney-shaped, and occupy a very large part of the abdominal cavity. Their importance may be estimated from the fact that their weight at this time is equal to a little over of that of the entire body-a proportion which is seven or eight times as large as that of the kidneys, in the adult condition. There are, indeed, at this period, only three organs perceptible in the abdomen, viz., the liver, which has begun to be formed at the upper part of the abdominal cavity; the intestine, which is already somewhat convoluted, and occupies its central portion; and the Wolffian bodies, which project on each side the spinal column.

Fig. 227.

FETAL PIG, % of an inch

long; from a specimen in the

author's possession. 1. Heart.

2. Anterior extremity. 3. Pos

terior extremity. 4. Wolffian body The abdominal walls

have been cut away, in order to show the position of the

Wolffian bodies

The Wolffian bodies, in their intimate structure, resemble very closely the adult kidney. They consist of secreting tubules, lined with epithelium, which run from the outer toward the inner edge of the organ, terminating at their free extremities in small rounded dilatations, or culs-de-sac. Into each