THE DEVELOPMENT OF VERTEBRATA. 249 



(by far the greater part of it) to the frog's yolk-cells. But 

 this latter part is not divided into cells, because the 

 enormous abundance of yolk prevents the protoplasm from 

 dividing completely. This is expressed by saying that the 

 segmentation of the fowl's egg is meroblastic (in contrast to 

 the holoblastic segmentation of Amphioxus and Rana). 



3. The Blastoderm. Presently, however, the proto- 

 plasm around the lower layer of nuclei becomes completely 

 marked off into cells, leaving below it the main mass of the 

 ovum, without any cell-structure, though probably with 

 some nuclei still present in it. This undivided mass may 

 from this time be conveniently called the yolk. The actual 

 cells form a small circular cap on the upper surface of the 

 yolk : this cap is called the blastoderm. 



The changes so far described take place while the ovum 

 is in the oviduct, and though the exact stage reached by 

 the time of laying varies, the blastoderm is always well 

 established. It may often * be seen as a circular patch 

 (g inch diameter) on the surface of the yolk when an 

 ordinary egg is broken into a dish. 



The fall of temperature that occurs when the egg is laid 

 and exposed to the air stops further development until 

 incubation begins. This is normally effected by the warmth 

 of the hen's body as it " sits," but artificial means are 

 quite as successful. 



4. Epiblast and Primitive Hypoblast. As seen from 

 the surface, the blastoderm very soon shows division into 

 two areas a central area pellucida and an outer area opaca 

 (fig. 127), whose names express their difference of appear- 

 ance. In a section (fig. 126) we find the explanation of the 

 difference to lie in the thickness of the lower layer of cells, 

 which is several cells thick near the margin, but only one 

 cell thick in the area pellucida. In the latter region it is 

 also more distinctly separated from the yolk, by the forma- 

 tion of a sub-germinal cavity (which must not be confused 

 with the segmentation cavity between the upper and lower 



* It must be borne in mind that under domesticated conditions 

 hens may lay unfertilized eggs. 



