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. Asa 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 
Fic. 272.—Mammalian ovum.— owt is further prolonged during 
‘After Hertwig. actation (Ryder); on the other 
: hand, the chance of survival is 
ov., Ovum; /,, follicular capsule; /z., much increased, and the maternal 
follicle cells ; fc., follicle cells form- ° 26 Daan f ‘ 
ing discus proligerus; £2, cavity Sacrifice finds its 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- 
