I02 THE EVOLUTION OF SEX. 



question as to the extent in which the yolk ever shares in 

 directly contributing to embryonic structures. The possibility 

 of distinguishing between formative protoplasm 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 yolk material is diffusely distributed. Then 

 the OYum. undergoes complete segmentation, {b.) In the frog's 

 ovum, on the other hand, there is a large proportion of yolk, 

 which has especially accumulated in the lower hemisphere of 

 the cell, while the darker half includes the truly formative pro- 

 toplasm. 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, (c.) A dis- 

 tinct mode of yolk arrangement occurs in arthropods (crusta- 

 ceans, insects, &c.), where the centre, 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 yolk. 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 yolk present, and its diffuse, polar, or 

 central arrangement, are associated with striking differences in 

 the degree and symmetry of the segmentation. 



§ 4. Composite Ova. — "We have emphasised 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 flat worms there occur what have been 

 called compound ova. A minute single cell arises, as usual, in the ovar)', 

 but in the course of its somewhat intricate histoiy this becomes associated 

 with several nutrient cells derived from the yolk-gland. These sur- 

 round 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 

 fertilised, and that it contains all the formative protoplasm. Those 

 that surround it are wholly nutritive ; they eventually break up, and are 

 absorbed. 



In other cases, especially in insects, the ovum grows rich at the expense 

 of neighbouring cells, which are sacrificed to its nutritive equipment. But it 

 is evident enough that a cell remains a cell, however many of its neighbours 

 it may happen to absorb. 



§ 5. Egg E^ivelopes. — The ovum starts as a naked cell, but generally 

 becomes furnished with ensheathing envelopes. The exact history of the 

 egg-membranes and sheaths is a very complex matter. Only the most 



