572 



STRUCTURE OF VERTEBRATA 



fc- 



is fitted, tends to prevent a speed}' establishment of the adult form. 

 When much yolk is present, it usually forms a hernia-like yolk-sac, 

 hanging down from the embryonic gut. As a further consequence, we 

 may notice the tendency to the production of embryonic organs useful 

 only during embryonic life. We must consider the formation of an 

 organic connection between mother and unborn young as a further 

 step in the same direction as the acquisition of yolk. This is hinted 

 at in some Fishes and Reptiles, but culminates in the placental 

 Mammals. It may be looked at in two different ways. On the one 

 hand, the diversion of the nourishment from the ovary, during the 

 period of gestation, tends to starve the remaining ovarian ova, and 

 this check to fertility is further prolonged during lactation (Ryder) ; 

 on the other hand, the chance of survival is much increased, and the 

 maternal sacrifice finds its justification in the increased specialisation 



of the offspring. 



In accordance with the effect of 

 the presence of yolk as noted above, 

 we l^nd that segmentation is total 

 (holoblastic) in the ova of the 

 lamprey, the sturgeon, Ceratodus, 

 Amphibians, and all Mammals 

 except the Monotremes. In the 

 t)va of FJasmobranchs, Teleosteans, 

 Reptiles, Birds, and Monotremes, 

 the activity of the protoplasm is 

 not sufficient to overcome the 

 inertia of the yolk, and segmenta- 

 tion is partial (meroblastic). 



Similarly we find that a gastrula 

 is formed, in part at least, by dis- 

 tinct invagination in the develop- 

 ment of the lamprey, the sturgeon, 

 and Amphibians (though the oc- 

 currence of invagination has been denied for the frog) ; it is more 

 modified in Teleosteans and Elasmobranchs, whose ova have more 

 yolk ; it is much disguised in Sauropsida and Mammals. 



• Most Vertebrates lay eggs in which the young are hatched 

 outside of the body, and to all these forms the term oviparous 

 is applied. In some sharks, a few Teleosteans, some tailed 

 Amphibians, a few lizards and snakes, the young are hatched 

 before they leave the body of the mother. To these cases the 

 awkward term ovo-vkiparous is applied, but there is no real 

 distinction between this mode of birth and that called ovi- 

 parous, and both may occur in one animal {e.g. in the grass- 

 snake) in different conditions. In the placental Mammals 

 there is a close organic connection between the unborn 

 young and the mother, and the parturition in this case is 

 usually called viviparous. But all the three terms are bad. 



Fig. 323. — Mammalian ovum. — 

 After Hertwig. 



ov., Ovum ; /., follicular capsule ; fz., 

 follicle cells ; f.c, follicle cells form- 

 ing discus proligerus ; /./., cavity 

 occupied by liquor foUiculi. 



