2 3 o THE EVOLUTION OE SEX. 



aphides are usually viviparous, while the fertilized eggs are laid as 

 such. 



VI. Early Nutrition. — The early nutrition of the embryo, and 

 even larva, is in most cases an absorption of the legacy of yelk mate- 

 rial, which is probably richest in the eggs of birds. The tadpole of 

 the frog grows and exerts itself for some time before it begins to feed 

 at the expense of this inheritance of yelk. Later on, in the frog 

 division of amphibians, the growth of new structures appears to be 

 provided for by the nutritive absorption of the tail, the larva literally 

 living upon itself. The same is true in the elaborate metamorphosis 

 of echinoderm larvae. In many cases, the cells of the embryo, inde- 

 pendently and actively, devour the yelk and other available material, 

 doing so after the amoeboid fashion technically known as intra-cellular. 

 At the same time, osmotic currents may more passively effect the 

 like result. In the whelk and related forms, a curious cannibalism is 

 well known to occur among the crowd of embryos inclosed within a 

 common capsule. The stronger and older devour the younger and 

 weaker, — a struggle for existence happily of exceptional precocious- 

 ness. In the higher vertebrates (above amphibians), foetal membranes- 

 — amnion and allantois — are developed, in addition to the yelk-sac 

 which incloses the yelk. Of these the amnion is mainly protective, 

 and the allantois at first almost wholly respiratory. But in birds (and 

 probably to a slight extent in reptiles) the allantois begins to assume 

 nutritive functions, assisting in the absorption of the yelk. In placental 

 mammals, however, a nutritive function becomes paramount, the allan- 

 tois forming the greater part of the embryonic side of the placenta. 

 The yelk-sac is here virtually yelkless, but in lower orders may absorb 

 nutriment as it did in birds, though from a different source — the 

 maternal wall. In most cases, however, what was incipient on the part 

 of the yelk-sac, in the exceptional elasmobranchs and lizards already 

 mentioned, becomes the emphatic function of the allantois, — namely, 

 the establishment of a vascular or nutritive connection with the wall or 

 the maternal uterus. By this means, though no drop of blood ever 

 passes from mother to offspring, a very intimate osmotic transfusion is 

 effected. 



VII. Lactation. — If menstruation be a means of getting rid 01 

 anabolic surplus, in absence of the foetal consumption, lactation is still 

 more an anabolic overflow, adapted to, though not of course originally 

 caused by the offspring's demands. It is at the same time evident 

 enough, and easily verified by the histologist, that in actual occurrence 

 both processes are katabolic, involving cellular disruption and death. 

 That peculiar liability of these uterine and mammary tissues to disease, 



