494 



STRUCTURE OF VERTEBRATA. 



0"J. 



Hermaphroditism occurs as a normal state in Tunicata, most of which 

 are first functionally female and then male (protogynous) ; in Myxine 

 ((/.v.), which is first male and then female (protandrous) ; in some 

 species of the Teleostean genera Chrysophrys and Serramts, of which 

 the latter is regularly self-fertilising ; and in a solitary Batrachian. It 

 occurs casually in some Selachians, in the sturgeon, in about a score of 

 Teleosteans, e.g. cod, in various Amphibians, and more rarely in 

 Amniota. There are also embryological facts which suggest that the 

 embryos of higher Vertebrates pass through a state of hermaphroditism 

 before the unisexual condition is reached. On these grounds it has 

 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 pro- 

 toplasm, 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 hernia-like yolk-sac, hang- 

 ing down from the embryonic gut. 

 As a further consequence, we may notice the tendency to the produc- 

 tion 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 special- 

 isation 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 



FIG. 237. Mammalian ovum. 

 After Hertwig. 



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

 follicle cells ; f.c., follicle cells form- 

 ing discus proligerus ; f.L, cavity 

 occupied by liquor folliculi. 



