THE YOLK-SAC. 65 



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 usually 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 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 transition of the area opaca into the 



FIG. 31. 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. 



area vitellina is marked by a considerable accumulation 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 assimilation is going on, and it can be easily seen that it is 

 most advanced in those cells which are nearest the embryo and least advanced in 

 those cells which are nearest to the germinal wall. In mammals the area pellucida 

 is well marked and resembles that of birds. The area opaca has well-defined 

 cylinder cells (Fig. 31) which have rounded ends, but are much smaller than in 

 birds and contain very little yolk material. Cells of this character extend over also 

 what we should call the area vitellina, which does not present the special features 

 which it has in birds, for the reason that the yolk in mammals is so small in 

 amount and the yolk-sac, therefore, is hollow. Later on the cells pass through 

 degenerative changes, which need to be more exactly studied. In man the degen- 



