SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 249 



larvae. In many cases, the cells of the embryo, independently 

 and actively, devour the yolk 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 enclosed within a common capsule. The stronger 

 and older devour the younger and weaker, — a struggle for 

 existence happily of exceptional precociousness. In the 

 higher vertebrates (above amphibians), foetal membranes — 

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

 sac which encloses the yolk. 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 yolk. In placental mammals, however, 

 a nutritive function becomes paramount, the allantois forming 

 the greater part of the embryonic side of the placenta. The 

 yolk-sac is here virtually yolk-less, 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 yolk-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 of the maternal uterus. By 

 this means, though no drop of blood ever passes from mother 

 to offspring, a very intimate osmotic transfusion is effected. 



§ 6. Lactation. — If menstruation be a means of getting rid 

 of anabolic surplus, in absence of the foetal consumption, lacta- 

 tion 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 kata- 

 bolic, involving cellular disruption and death. That peculiar 

 liability of these uterine and mammary tissues to disease, 

 which furnishes the most tragic possibihties of the life of 

 woman, becomes thus less mysterious. We can understand more 

 readily the association of such diseases with much of what we 

 are pleased to generalise as civilisation, and view more hope- 

 fully the possibilities of their enormous diminution by the 

 rational hygiene of civilisation properly so-called. 



The milk or mammary organs are modified skin-glands. 



