9 6 THE EVOLUTION OE SEX. 



(c) In the third place, and this is the rarest form, the egg-cell 

 acquires a store of food-material from a special yelk-gland, as in 

 many of the lower "worms." But we have already pointed out that 

 this yelk-gland is usually interpreted as a degenerative portion of the 

 essential organ. 



The yelk, gained in the above ways, is more or less readily dis- 

 tinguished from what is often called the formative protoplasm. Out 

 of the latter the embryo is built up, while the yelk has for the most 

 part only a secondary and nutritive rule. We can not, of course, 

 enter here into the difficult embryological question as to the extent in 

 which the yelk ever shares in directly contributing to embryonic 

 structures. The possibility of distinguishing between formative pro- 

 toplasm and the nutritive material, depends on the quantity of the 

 latter that is present, and on the way in which it is disposed, (a) When 

 there is not much of it, as in the small ova of mammals and many 

 invertebrates, the yelk-material is diffusely distributed. Then the 

 ovum undergoes complete segmentation, (fr) In the frog's ovum, on 

 the other hand, there is a large proportion of yelk, which has especially 

 accumulated in the lower hemisphere of the cell, while the darker half 

 includes the truly formative protoplasm. In this case too the egg 

 divides as a whole, but the divisions go on much more rapidly in the 

 upper hemisphere, and it is there that the embryo is really formed. 

 <Y) A distinct mode of yelk-arrangement occurs in arthropods 

 (crustaceans, insects, &c), where the center, not a pole, of the 

 ovum is occupied by the nutritive material. In this case the forma- 

 tive protoplasm divides round about the nutrient core, (d) In the 

 majority of fishes, in reptiles, and in birds, the eggs show a much 

 more marked polar accumulation of yelk. On the top of a large 

 mass of nutritive material, the specifically lighter formative protoplasm 

 lies like a tiny drop, and in those cases the division of the ovum is 

 very partial, — that is, it is mainly restricted to the upper formative 

 region. It is thus to be noted that the quantity of yelk present, and 

 its diffuse, polar, or central arrangement, are associated with striking 

 differences in the degree and symmetry of the segmentation. 



IV. Composite Ova. — We have emphasized the fact that the ovum must be 

 regarded as a single cell. To this a definite but pedantic objection has been 

 raised. In some parasitic flatworms there occur what have been called com- 

 pound ova. A minute single cell arises, as usual, in the ovary, but in the 

 course of its somewhat intricate history this becomes associated with several 

 nutrient cells derived from the yelk-gland. These surround the original ovum, 

 so that the whole now consists of several cells. But it must be noticed that only 

 the central cell -the ovum proper — is fertilized, and that it contains all the 

 formative protoplasm. Those that surround it are wholly nutritive ; they event- 

 ually break np, and are absorbed. 



