REPRODUCTIVE SYSTEM. 467 



often been suggested that the original Vertebrate animals were 

 hermaphrodite. 



The quantity of yolk present in the egg varies very greatly in 

 Vertebrates, and its presence or absence exercises a profound influence 

 upon the processes of development. Following Hertwig, we may notice 

 that the presence of yolk has both a physiological and a morphological 

 effect. Physiologically, the presence of a store of nutriment enables 

 the developmental process to be carried on uninterruptedly, and the 

 period of independent life to be postponed until more or less complexity 

 of organisation has been attained. Morphologically, the yolk acts 

 as a check to the activity of the protoplasm, and by substituting an 

 embryonic mode of nutrition for that for which the adult organism is 

 fitted, tends to prevent a speedy establishment of the adult form. 

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

 ing 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 un- 

 born 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, FlG> I9 8._Mamnialian ovura.- 

 dunng the period ot gestation, tends After Hertwig. 



to starve the remaining ovarian ova, , „. ' 



, ... , , . r 5?.., . r ,, ov.. Ovum ;/.. follicular capsule \ fz., 



and this check to fertility is further foU i c le cells \/.c, follicle cells foV 

 prolonged during lactation (Ryder) ; ing discus proligerus ; /.I., cavity 

 on the other hand, the chance of occupied by liquor folliculi. 

 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 find that segmentation is total (holoblastic) in the ova of the lam- 

 prey, the sturgeon, Ceratodus, Amphibians, and all Mammals except the 

 Monotremes. In the ova of Elasmobranchs, Teleosteans, Reptiles, 

 Birds, and Monotremes, the activity of the protoplasm is not sufficient 

 to overcome the inertia of the yolk, and segmentation is partial 

 (meroblastic). 



Similarly we find that a gastrula is formed, in part at least, by distinct 

 invagination in the development of the lamprey, the sturgeon, and 

 Amphibians (recently the occurrence 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 ovi- 



