514 STRUCTURE OF VERTEBRATA. 



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 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 acqui- 

 sition of yolk. This is hinted at in 

 some Fishes and Reptiles, but cul- 

 minates in the placental Mammals. 

 It may be looked at in two differ- 

 ent ways. On the one hand, the 

 diversion of the nourishment from 

 the ovary, during the period of 

 gestation, tends to starve the remain- 

 ing ovarian ova, and this check to 



After Hertwig. > 



m., Ovum; ; yC, follicular capsule ;./., much increased, and the maternal 



6f*S? ; ^5S{i2fVtS i < 35 sacri . fice **, it. justification in 



occupied by liquor folliculi. 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 dis- 

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

 and Amphibians (though 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- 



