736 



FORMATION OF THE EMBRYO. 



of rectangular masses arranged in longitudinal series, almost exactly 

 similar to each other, and separated by regular transverse divisions. 

 They resemble strongly in their appearance the simpler component parts 

 of a spinal column, and, in fact, form the basis out of which this struc- 



Fig. 252. 



TRANSVERSE SECTION OF THE CHICK-EMBRYO, at different stages of development. 

 Magnified 40 diameters. 



I. On the second day of incubation. 



II. Between the second and third days. 



III. On the third day. 



IV. On the fourth day. 



1. Medullary groove. 2. Medullary canal. 3. External blastodermic layer. 4. Internal 

 blastodermic layer. 5. Space of separation between the two laminae of the blastoderm; 

 future peritoneal cavity. 6. Chorda dorsalis. 7. Primitive vertebrae. 8. Aorta. 9. Cavity 

 of the intestine. (His.) 



ture will afterward be developed. But they do not represent imme- 

 diately and exclusively the bodies of the vertebrae. They are to serve, 

 not only for the formation of these organs, but also for that of the spinal 

 muscles on the one hand and of the muscular layer of the aorta on the 

 other. They are therefore known as the primitive vertebras. In a 

 transverse section of these bodies (Fig. 252, 7 ) there is an evident dis- 



