go6 General Notes. | September, 
` ever, there is a manifest tendency for that structure to increase 
rapidly in size owing to the imbibition of fluid with which the 
blastoccel becomes more and more distended, this increase in size 
at the same time being aided by the division of the cells entering 
into the formation of the walls of the blastoccel. The greater 
part of the walls of this vesicle are finally metamorphosed by a 
process of folding off and ingrowth of the embryo into the vesi- 
cle by invagination, into a respiratory apparatus and secondary 
system of envelopes, a portion of which also takes part more or 
less extensively in the absorption of pabulum from the surround- 
ing uterine surfaces which may be more or less completely re- 
flected around the embryo and its vesicle, to be finally cast off at . 
birth together with those parts of the vesicle derived from the 
ectoblast, which are also deciduous. The vesicle also tends, with 
a few exceptions, to thrust out hollow villi, which dip into pits in 
the uterine mucosa. ese may arise locally or all over the ves- 
icle, and reach their fullest development when the chorion has 
been formed, when the greater part of the surface of the vesicle 
acquires a shaggy covering of villi, into which vascular loops 
from the allantois are insinuated, over a restricted area internally 
or over its whole surface. These then become more or less com- 
pletely insinuated into vascular uterine crypts into the constitu- 
tion of which a decidua may or may not enter. 
t will, I think, be obvious to any one, that if an oviparous 
paratherian form were to have the eggs which it produces so 
modified as to lose the shell, yolk and albuminous and fibrous 
envelopes, leaving only the naked endocyemate ovum to be re- 
tained near the outlet of the oviduct, the wall of which would 
then become thickened so as to form a specialized uterine dilata- 
tion, the conditions for a realization of the eutherian mode o 
viviparous development would be present. In this way, no doubt, 
the peculiarly specialized mode of mammalian development 
arose. 
Objections may be urged against the position I have assigned 
to the Amphibia and Marsipobranchii as well as to the names 
given to the groups, and to the stress laid upon the physiological 
aspects of development and their importance not only in taxon- 
omy but also in tracing the mode of the evolution of particular 
grades of development. ; 
The form of the placenta seems to depend upon several factors: 
(1) The early or late attachment of the blastodermic vesicle to the 
