306 



AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



fate. The other four are termed inner cells, for a similar reason. 

 Both inner and outer cells continue to divide, but the latter 

 increase more rapidly and grow over the others. At the close 

 of segmentation, which takes place in the Fallopian tube, the 

 blastula consists (Fig. 92) of a layer of clear outer cells, which 

 cover a central mass of granular inner cells, except at one point. 

 The embryo is still invested by the zona radiata, external to 

 which is an albuminous coating, the zona pellucida, received from 

 the wall of the Fallopian tube. The changes that follow take 

 place in the uterus, and lead to the formation of the embryonic 

 layers. 



The outer cells first of all grow completely over the inner cells, 

 and then increase and become flattened to form the blastocyst 

 (blastodermic vesicle), an internal cavity forming meanwhile, 

 which gets larger and larger. The blastocyst is a delicate 

 vesicle, the walls of which are mainly formed by outer cells, to 

 which the inner cells adhere over a small internal area. It 

 continues to increase in size, and the inner cells gradually extend 

 at the edges. In the centre, however, of the patch of inner cells 

 a circular thickening appears (Fig. 93), the embryonic area, 

 corresponding to the area pellucida of the Chick. The inner 

 cells here become divided into an upper layer of rounded cells 

 and a lower layer of flattened cells. The former unite with the 

 outer cells to form the ectoderm (epiblast) of the embryonic area, 

 while the latter constitute the endoderm (hypoblast) of the same 

 region. Outside this the epiblast and hypoblast are formed by 

 the outer and inner cells respectively. 



The mesoderm (mesoblast) is first formed in the embryonic 

 area, and has a double origin similar to that described for the 



