1885.] Embryology. 905 
dd, Placenta deciduate, discoidal or exceptionally non-decidu- 
ate and diffuse as in the Lemuride. Rodentia, Insectivora, Chei- 
roptera, Primates. 
uch remains to be learned of the earliest stages of the forma- 
tion of the placenta, especially in the Primates. In Talpa, Heape 
has found a rudimentary “träger” or suspensor developed. In 
some of the Rodentia the embryonic mass is precociously invag- 
inated into the blastoccel, and the amniotic cavity is formed in 
the most extraordinary manner, or by a sort of vacuolization or 
accumulation of fluid (liquor amnii) in the midst of the mass of 
undifferentiated embryonic cells. This occurs in Mus, Arvicola 
and Cavia, according to Selenka. In these forms the blastodermic 
vesicle also becomes adherent to the uterine epithelium at a very 
early stage, and the suspensor is very markedly developed in the 
three forms mentioned. The precocious invagination of the un- 
differentiated embryonic mass of cells into the blastoccel leads, in 
the Rodentia, to an apparent inversion of the embryonic layers. 
These forms have therefore attained the most specialized mode of 
development known amongst Mammalia, so that, judged by the 
standard of embryology alone, they would rank higher than the 
Primates. 
The foregoing scheme illustrates in a very striking manner the 
way in which complication after complication has been added to 
the developing germ, starting with a simple blastula developed 
by total cleavage in Branchiostoma ; the-next step in the progress 
of embryonic specialization is that seen in the amphibian and mar- 
sipobranchian embryo, in which a distinct neurenteric canal is also 
developed, and in which the neurenteron is continued into the 
enteric cavity, which itself traverses the segmented vitelline mass 
longitudinally along its upper half. In the next grade of special- 
ization, or that represented by the Ichthyes of this arrangement, 
the vitellus remains unsegmented for a long time, and is practi- 
cally excluded from sharing in the formation of the enteric walls, 
but the embryo is sessile in the greater number of species em- 
braced in this series, and while only a portion of the blastodermic 
area leads to the differentiation of an embryo, no part of the ecto- 
last is ever so folded off to form provisional organs such as the 
