316 THE MAMMALIAN EMBRYO. [CHAP. 



part of it remains however thicker, and is constituted 

 of two rows of cells, while the peripheral part, the outer 

 boundary of which is irregular, is formed of an imperfect 

 layer of amoeboid cells which continually spread farther 

 and further beneath the outer layer. The central thick- 

 ening of the inner layer forms an opaque circular spot 

 on the blastoderm, which constitutes the commencement 

 of the embryonic area. 



The formation of the layers. The history of the 

 stages immediately following, from about the com- 

 mencement of the fifth day to the seventh day, when a 

 primitive streak makes its appearance, is not perfectly 

 understood, and has been interpreted very differently by 

 various observers. The following account must there- 

 fore be considered as a tentative one. 



About five days after impregnation the cells of the 

 inner mass in the embryonic area become divided into 

 two distinct strata, an upper stratum of rounded cells 

 adjoining the flattened outer layer and a lower stratum 

 of flattened cells. This lower stratum is the true hypo- 

 blast (Fig. 97). At the edge of the embryonic area the 

 hypoblast is continuous with a peripheral ring of the 

 amoeboid cells of the earlier stage, which now form, 

 except at the edge of the ring, a continuous layer of 

 flattened cells in contact .with the outer layer. During 

 the sixth day the middle layer becomes fused with the 

 outer layer, and gives rise to a layer of cells which are 

 columnar and are arranged in the rabbit in a single 

 row (Fig. 98). They form together the true epiblast of 

 the embryonic area. 



At this stage therefore the embryonic area, which is 

 circular, is formed throughout of two single layers of 



