THE EGG-CELL OR OVUM. 



lOT 



surviving competitor among a crowd of surrounding cells, which 

 to start with were all potential ova. (c.) In the third place, and 

 this is the rarest form, the egg-cell acquires a store of food- 

 material from a special yolk gland, as in many of the lower 

 "worms." But we have already pointed out that this yolk- 

 gland is usually interpreted as a degenerate portion of the 

 essential organ. 



B 



J) 



The relation between the disposition of the yolk and the mode 

 of segmentation : — A, diffuse yolk, e.^., sponge ; B, polar, 

 c.£:, frog ; C, central yolk, e ^^., crayfish ; D, predomin- 

 ant, e.£:, bird : — A', lotal and equal segmentation ; B', 

 total and unequal; C, peripheral ; D', partial segmentation. 



The yolk, gained in the above ways, is more or less readily 

 distinguished from what is often called the formative protoplasm. 

 Out of the latter the embryo is built up, while the yolk has 

 for the most part only a secondary and nutritive 7v/e. We 

 cannot, of course, enter here into the difficult embryological 



