914 GENERATION. 



a single opening (#), which is formed by the union of the two openings (</, g) in Figs. 

 294 and 295, and this is the abdominal aorta, which has here become single. The two 

 openings (h, h) represent a section of the pleuro-peritoneal cavity. The outer wall of 

 this cavity is the outer visceral plate, which is developed into the muscular walls of the 

 abdomen. The lower and inner wall is the inner visceral plate, which forms the main 

 portion of the intestinal walh The outer wall is the outer layer of the intermediate 

 membrane, and the inner wall is the inner layer of the same membrane. The two round 

 orifices (*', i) are sections of the Wolffian ducts. 



The figures we have just described, it must be borne in mind, represent transverse sec- 

 tions of the body of the chick, made through the middle portion of the abdomen. In our 

 explanations of these figures, we have not adhered absolutely to the text of Brticke, but 

 have made use of the very elegant semi-diagrammatic illustrations by Waldeyer, whose 

 explanations are remarkably clear and satisfactory. Our explanations, however, particu- 

 larly those of Fig. 296, are sufficiently extended to enable us to study the development 

 of special organs. The posterior parts, it is seen, are developed first, the situation of the 

 vertebral column being marked soon after the enclosure of the neural canal by the verte- 

 bral plates ; and, at about the same time, the two aortae make their appearance, with the 

 first traces of the pleuro-peritoneal cavity. The next organs in the order of development, 

 after the vascular system, are the Wolffian bodies, which are so large and important in 

 the early life of the embryon. The intestinal canal is then a simple groove, and the em- 

 bryon is entirely open in front. Were we now to follow the process of development far- 

 ther, we should see that the visceral plates advance and close over the abdominal cavity, 

 as the medullary plates have closed over the neural canal. Thus there would be formed a 

 closed tube, the intestine, lined by the thin, internal blastodermic membrane, the walls of 

 the intestine being formed of the inner layer of the intermediate membrane. This would 

 bring the external layer of the intermediate membrane around the intestine to form the 

 muscular walls of the abdomen, the cavity (Fig. 296, ^, A) being the peritoneal cavity, and 

 the external covering being the external blastodermic membrane. At this time, the great 

 Wolifian bodies lie next the spinal column, between the intestine and the abdominal 

 walls, with the single abdominal aorta situated behind the intestine. 



Development of the Skeleton, Muscular System, and Skin. 



Chorda Dorsalis. One of the earliest structures observed in the developing embryon ' 

 is the chorda dorsalis. This is situated beneath the neural canal and extends the entire 

 length of the body. It is formed of a cord of simple cells, and marks the situation of the 

 vertebral column, though itself it is not developed into the vertebras, which grow around 

 it and encroach upon its substance, until it finally disappears. This structure has been 

 very minutely described by Robin, under the name of the notocorde. In many mam- 

 mals, the notocorde presents a slight enlargement at the cephalic extremity, which ex- 

 tends to the auditory vesicles and it is somewhat diminished in size at the caudal extrem- 

 ity. By the sides of this cord, are the masses of cells which are eventually developed 

 into the vertebra. The vertebrae, as they are developed, are formed of temporary car- 

 tilaginous structure, gradually extending around the chorda dorsalis, which then occupies 

 the axis of the spinal column. Between the bodies of the vertebras, the chorda dorsalis 

 presents regular enlargements, surrounded by a delicate membrane. As ossification of 

 the spinal column advances, that portion of the chorda dorsalis which is surrounded by 

 the bodies of the vertebrae disappears, leaving the enlargements between the vertebrae 

 distinct. These enlargements, which are not permanent, are gradually invaded by fibrous 

 tissue, their gelatinous contents disappear, and the intervertebral disks, composed of fibro- 

 cartilaginous structure, remain. These disks are permanent between the cervical, the 

 dorsal, and the lumber vertebrae ; but they eventually disappear from between the dif- 

 ferent parts of the sacrum and coccyx, as these are consolidated, this occurring, in the 



