1919.] Intra-uterine Embryos of Elasmobranchs. 151 
yolk-sac type of the others. We have in the course of our studies 
been able to distinguish three distinct grades. 
(i) In Scoliodon sorrakowah and S. palasorrah we meet with 
the least modified type. Here the typical rounded or slightly 
ovoid yolk-sac develops at its free extremity a number of small 
protruberances which become embedded in the uterine wall 
and form a very simple type of placenta. The connection be- 
tween the embryo and the mother is not of a very intimate 
character. 
(ii) The placenta of Mustelus laevis and of a species of 
Carcharias, figured by Muller, is of a more advanced type. 
Here we find a distinct placenta-like interdigitation of folds of 
the yolk-sac, the villi-like projections fitting into correspond. 
ing depressions in the mucous membrane of the uterus, simi- 
lar to the cotyledons of the placenta of the Ruminants in 
Mammals. 
(iii) The more advanced arborescent type of placenta, 
many grades of which also occur, is to be seen in a large num- 
ber of Selachii. When fully evolved it is a fairly large spread- 
of the placental cord and the remains of the yolk-sac. _ The 
blood-vessels in the placental-cord divide again and again to 
highly vascular portion of the uterine wall. The food in these 
forms is obtained by the embryo from the mother through the 
vascular system. 
Placental cord and its appendages.—The placental cord of 
the Selachii results from a transformation of the yolk-stalk. 
When the yolk in the yolk-sac is exhausted, and the placenta is 
being formed from the yolk-sac the connection between the 
yolk-sac and the alimentary canal of the embryo is cut off. 
aplacental forms are the same. ; 
formation of the yolk-stalk into placental cord takes place 
th the modification of the yolk-sac into the placenta. It 
may be remarked here that in their descriptions of the embryos, 
some authors have designated the yolk-stalk of the aplacental 
Batoids as the umbilical cord. This name, in view of the fact 
that in the aplacental forms no placental connection 1s ever 
developed between the foetus and the maternal uterine tissue, 
and further the origin of the aplacental forms from oviparous 
and not the placental viviparous forms, is quite a wrong name. 
The walls of placental cords of the sharks are, ina number of 
