GENERAL HISTORY OF THE EMBRYO. 



363 



closely, as just noticed, with the area pellucida of a hen's egg 

 about the sixteenth hour of incubation. 



The formation of the rabbit embryo is also effected in very 

 similar fashion to the chick. The embryonal area increases in 

 size, especially by growth at its anterior end. Immediately in 

 front of the primitive streak a neural groove (Fig. 141, XG) is 



AN' 



E' 



si 



FIG. 145. A Rabbit Embryo at the end of the ninth day. The entire blasto- 

 dermic vesicle is represented, with the embryo in situ, as seen from the 

 dorsal surface. (Cf. Fig. 146, which represents an embryo of the same age 

 in sagittal section.) x 10. 



AN', proamnion. BM, mid-brain. E', horse-shoe shaped patch of thickened epiblast, 

 by which the blastodenuic vesicle is attached to the wall of the uterus (cf. Fig. 169). 

 MS, mesoblustic somite or protovertebra. R, right half of heart. SI, sinus teriuiualis. 

 TA, allantois. 



formed, bordered by neural folds, XF, which speedily unite, con- 

 verting the groove into a tube. This tube becomes the central 

 nervous system, and in its anterior or cerebral part the several 

 brain vesicles are early established (Fig. 145). 



By means of head, tail, and side folds the embryo is con- 



