34 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



there can be no excuse for mistaking them. In Cyntliia, indeed, these differences 

 can be easily recognized in the living egg, the yolk being slate-gray and the proto- 

 plasm colorless or yellow (figs. 28, 32, 37, 38). The yolk spherules are not scattered 

 through the cytoplasm, but the limits of the yolk and cytoplasm are sharp and dis- 

 tinct. In the cells at the yolk pole the cytoplasm is limited to a small area around 

 the nucleus; at the protoplasmic pole the cytoplasm occupies a large part of the 

 cell, the yolk being limited to the inner ends of the cells. This is seen especially 



,* * 



xvm 



Figs. XVII-XX. Actual sections of eggs of Cynthia partita; Figs. XVII anil XIX in the median 

 plane, Figs. XVIII and XX in a transverse plane. Figs. XVII and XVIII represent a 20-24 cell stage; 

 Figs. XIX and XX a 32-44 cell stage. Unshaded portions of cells represent clear protoplasm ; closely 

 crowded spheres, the yolk ; minute spherules, the yellow protoplasm of the crescent. The clear protoplasm 

 is located chiefly in the cells of the animal half of the egg (ectoderm) ; it is also found in the crescent 

 cells (mesoderm) 1 15; =, B 6! , B 6 '3) and neural plate cells (A7-4, upper half of A 6 -=0 of the lower hemisphere. 

 The remaining cells of the lower hemisphere, (endoderm B5- 1 , B 6 - 1 , A 6,1 , A fc -3, and chorda, A<-3 and lower 

 half of A 6-2 ) are filled with yolk ; yolk is also found in the central ends of all the other cells. The yellow 

 protoplasm is limited to the crescent cells and to a single pair of cells of the upper hemisphere (bs-3). In 

 Fig. XVII the cells at the two poles are approximately equal in height ; in Fig. XVIII the cells at the 

 animal pole are flatter, probably owing to the fact that they are dividing ; in Figs. XIX and XX the cells 

 at the animal pole are columnar, those at the vegetal pole flattened. The polar bodies are actually pres- 

 ent where they are represented. 



such sections are shown in text figures 



well in actual sections taken in the vj:'s axis 

 XVII to XXIV, and it can there be seen that the cytoplasm is largely found in those 

 cells which lie on that side of the egg where the polar bodies are found, while the 

 cells at the opposite pole are almost entirely filled with yolk. These yolk-laden 



