248 ESSENTIALS OF ZOOLOGY 



coating of albumen. Segmentation results in the formation 

 of a ball of cells which differentiates into an outer envelope and 

 a central group of cells. The outer envelope is the serosa, 

 which is early developed to attach the embryo to the uterine 

 wall, and the central cells become cut off from the vegetable 

 pole of the outer layer by a split which is rapidly increased 

 into a cavity distended by a fluid. The central cells remain- 

 ing attached to the animal pole are resolved into the two 



Amniotic cav. Ed. End. Cyt. Plasmodioblast 



\ \ 



Yolk sac 



Ectoderm 

 Endoderm 



Endoderm 



Ect. (serosa) 

 Yolk-sac vesicle 



FIG. 127. Diagrams to illustrate the formation of the primitive 

 amnion of bats, etc. (after van Beneden), and the superficial 

 position of the embryonic ectoderm in rabbits, etc. 



layers of the embryonic area. The upper layer is the ectoderm, 

 and it is put into communication with the ectoderm of the 

 serosa; and the lower layer is the endoderm, and at first is 

 restricted to the embryonic area, but soon spreads out by 

 cell multiplication to form a flat layer lining the vesicle. The 

 products of segmentation are thus rapidly resolved into an 

 outer serosa and an embryonic area, or rather, an area pellucida 

 consisting of two layers. Furthermore, the endoderm has 

 grown to the extent to enclose a cavity. The subsequent 

 procedure demonstrates that this cavity comprehends the 

 enteron below the embryonic area and the yolk sac. The 

 cavity above, roofed in by the serosa, is a proamniotic cavity 



