LIMITATION IN LOWER VERTEBRATES 139 



instance, in the common smooth-hound, in many rays and in sharks, 

 the egg-case is very thin and deUcate, the quantity of food-yolk in 

 the egg is much less, and the eggs are retained in the body of the 

 mother, lodged in special expansions of the oviducts, until the 

 embryos have hatched and grown to young fish. The walls of 

 these sacs form long filaments, supplied with blood from the vessels 

 of the mother and serving for the nutrition of the young. In some 

 cases this secretion is swallowed by the embryo in the same way as 

 in the viviparous blenny. In other cases, the nutritive filaments 

 of the mother are arranged in a pair of bundles, one of which is 

 thrust through each spiracle of the embryo into its alimentary 

 canal, where the nutritive secretion is taken up. There is a still 

 higher development of this mode of maternal nutrition of the 

 embryo in some of the sharks, which recalls the embryonic stages 

 of mammals. The blood-vessels of the embryo grow out over the 

 yolk-sac, and absorb the yolk and use it for the growth of the 

 embryo. When the yolk-sac is exhausted of its contents, the blood- 

 vessels covering it grow out into tufts which intertwine with similar 

 vascular tufts arising from the tissues of the mother, and through 

 such a placental connection the blood of the mother conveys 

 nutrition to the embryo. 



Thus in various ways and by many different devices the number 

 in each brood of fishes becomes reduced in many species. Instead 

 of an enormous number being discharged to take their own chance, 

 a few are protected, sometimes fed, and only set free when they 

 have attained some degree of strength and capacity for protecting 

 themselves. As in lower animals, apart from its consequences in 

 better securing the maintenance of the species, this changed mode 

 of reproduction has a number of by-products. The growth of the 

 protected embryos, especially when they are supplied with much 

 food, either in the form of yolk or later on from the tissues of the 

 mother, is more direct and less a repetition of the ancestral history. 

 The instincts of the guardian parent or parents have become diverted 

 to new directions. Instead of being occupied throughout their 

 whole lives with their own individual concerns; the parents devote 

 some time and much trouble to matters which affect the safety of 

 the species rather than their own individual safety. They take 

 substances into the mouth, such as eggs, which would be good to 

 eat and yet do not eat them. They watch over, swim about with 

 and protect from others little moving creatures which a few weeks 

 before or a few weeks afterwards they would greedily devour. 



