4io 



AN INTRODUCTION TO ZOOLOGY 



the portion to be modified in connection with fixation and nutritive 

 requirements of the embryo. After reaching the uterus the blasto- 

 dermic vesicle enlarges fairly rapidly and in some species may 

 reach quite a large size. As its wall later takes part in the formation 

 of the placenta, which is among other things an organ of nutrition, 

 it comes to be termed the trophoblast or trophoblastic ectoderm. 

 The remaining internal group of cells, which is responsible for the 



V.W. 



FIG. 143. Lepus. .Segmentation of egg of rabbit. I. and II., first and 

 second cleavages ; III., section through morula ; IV., section through 

 fairly advanced blastodermic vesicle. After Assheton. 



A.L., albuminous layer ; C., cavity of blastoderm ; I., inner cell mass ; P.B., polar body ; 

 V.W., wall of blastodermic vesicle ; Z.R., zona radiata. 



production of the entire embryo and the whole entoderm, both 

 intra- and extra-embryonal, is distinguished from the former as the 

 formative or embryonal ectoderm. 



Two important processes now go on simultaneously, namely, 

 the formation of the germ layers and the embryonic rudiment, and 

 secondly the implantation or embedding of the vesicle in the uterine 

 wall. For convenience these two processes will be considered 



