90 ANIMAL PHYSIOLOGY. 
into completely segmenting (holoblastic), and those that under- 
go segmentation of only a part of their substance (meroblastic) ; 
but the processes are fundamentally the same. 
Provision is made for the nutrition, etc., of the ovum, when 
fertilized (odsperm) by the formation of yelk-sac and allan- 
tois; as development proceeds, one becomes more prominent 
than the other. The allantois may fuse with adjacent mem- 
branes and form at one part a condensed and hypertrophied 
chorion (placenta), with corresponding atrophy elsewhere. The 
arrangement of the placenta varies in different groups of ani- 
mals so constantly as to furnish a basis for classification. What- 
ever the variations in the structure of the placenta, it is always 
highly vascular; its parts consist of villi fitting into crypts in 
the maternal uterine membrane—both the villi and the crypts 
being provided with capillaries supported by a connective-tissue 
matrix covered externally by epithelium. The placenta in its 
different forms would appear to have been evolved from the 
‘diffuse type. 
The peculiarities of the embryonic membranes in birds are 
owing to the presence of a large food-yelk, egg-shell, and egg- 
membranes ; but throughout, vertebrates follow in a common 
line of development, the differences which separate them into 
smaller and smaller groups appearing later and later. The 
same may be said of the animal kingdom as a whole. This 
seems to point clearly to a common origin with gradual diver- 
gence of type. 
THE DEVELOPMENT OF THE EMBRYO ITSELF. 
We now turn to the development of the body of the animal 
for which the structures we have been describing exist. It is 
important, however, to remember that the development of parts, 
though treated separately for the sake of convenience, really 
goes on together to a certain extent; that new structures do not 
appear suddenly but gradually ; and that the same law applies 
to the disappearance of organs which are being superseded by 
others. To represent this completely would require lengthy de- 
scriptions and an unlimited number of cuts; but with the above 
caution it is hoped the student may be able to avoid erroneous 
conceptions, and form in his own mind that series of pictures 
which can not be well furnished in at least the space we have 
to devote to the subject. But, better than any abstract state- 
