DEVELOPMENT OF THE SKELETON. 393 



dermic 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 ab- 

 domen, the cavity (Fig. 39, A) being the peritoneal cavity, 

 and the external covering being the external blastodermic 

 membrane. At this time, the great "Wolffian bodies lie next 

 the spinal column, between the intestine and the abdominal 

 walls, with the single abdominal aorta situated behind the in- 

 testine. 



Development of the Skeleton, Muscular /System, and SJcin. 



Chorda Dorsalis. One of the earliest structures observed 

 in the developing embryon is the chorda dorsal is. This is sit- 

 uated 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 vertebrae, which grow around it and en- 

 croach upon its substance, until it finally disappears. This 

 structure has been very minutely described by Robin, under 

 the name of the notocorde. According to Robin, in many 

 mammals, the notocorde presents a slight enlargement at the 

 cephalic extremity, which extends to the auditory vesicles, and 

 it is somewhat diminished in size at the caudal extremity. 1 

 By the sides of this cord, are the masses of cells which are 

 eventually developed into the vertebrae. The vertebrae, as 

 they are developed, are formed of temporary cartilaginous 

 structure, gradually extending around the chorda dorsalis, 

 which then occupies the axis of the spinal column. Between 

 the bodies of the vertebrae, the chorda dorsalis presents reg- 

 ular 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 ver- 

 tebrae disappears, leaving the enlargements between the ver- 

 tebrae distinct. These enlargements, which are not perma- 



1 ROBIN, Memoire sur Devolution de la notocorde, Paris, 1868, p. 3. 



