THE YOLK-SAC. 



87 



appears in the fresh specimen darker, and this is called the area opaca, the second 

 region. The entoderm in this part consists of columnar cells (Fig. 35, c, and Fig. 

 36). In the chick the cells are high cylinder cells of somewhat irregular shape, 

 containing a loose network of granular protoplasm. The lower ends of the cells 

 are rounded and projecting, and have a well-marked border of dense protoplasm. 

 The nuclei are variable in size, but for the most part large, often three or four 

 times greater in diameter than the neighboring mesodermic nuclei. They usu- 

 ally have one, sometimes two, conspicuous nucleoli. The nuclei always lie 

 at the upper or basal ends of the cells, chiefly near one side of the cell. The cells 

 contain yolk-grains which appear to be undergoing resorption. Toward the area 

 pellucida the cells are smaller, the network of protoplasm closer, and the yolk- 

 grains are either absent altogether or, if present, small in size and few in number. 

 The transition to the thin entoderm of the area pellucida is quite abrupt. In 



Fig. 36. — Wall of the Yolk-sac in the Region of the Area opaca of a Rabbit Embryo of 



Thirteen Days. 

 V, Blood-vessels containing young red blood-cells, bl. mes, Mesoderm. 



the opposite direction the area opaca passes gradually, by changing its structure, 

 into the general mass of the yolk, or area vitellina, the third of the regions of the 

 yolk-sac, so called because it contains the bulk of the yolk material. The transi- 

 tion of the area opaca into the area vitellina is marked by a considerable accumu- 

 lation of cells which are arising from the yolk. This accumulation of cells 

 is called the germinal wall. It is the connecting-link between the epithelium 

 on the dorsal side of the entodermal cavity and the yolk or area vitellina, 

 which forms the ventral boundary of the cavity. If we follow successively the 

 stages, we find that the area pellucida grows at the expense of the area opaca, 

 and the area opaca at the expense of the area vitellina. These facts are to be 

 interpreted as phases in the process of the assimilation of the nutritive yolk. 

 The thin cells of the area pellucida are those in which the absorption of the yolk 

 has been completed. The larger cells of the opaca are those in which the assimil- 



