1886. | Physiology. 185 
With the increase in the size of the embryo, in these large-yolked 
forms, it is finally thrust down into a saccular involution of the 
blastoderm, the lips of the opening of which meet over the back 
of the embryo where they coalesce, the outer limb of the duplica- 
ture giving rise eventually to the serous envelope, and the inner 
to the roof of the amniotic cavity. In those types which have the 
primary layers apparently inverted, the rapid ingrowth of the sus- 
pensor precociously invaginates the germinal area inwards before 
the embryo is distinctly developed, so that it is not formed in its 
usual or normal position. These extreme modifications were not 
possible until after the loss of the food-yolk, after which a hollow 
blastodermic vesicle still continued to develop, filled with a thin 
albuminous or serous fluid instead of a dense yolk material. The 
tendency of the eutherian ovum to form a large, hollow blasto- 
dermic vesicle or blastula is doubtless an inheritance transmitted 
from a paratherian source. The bodily invagination of the whole 
embryo, and the more or less complete obliteration of the cavity 
of the umbilical vesicle by the rapid growth of the enlarging 
amnion, would be readily accomplished in the course of the 
development of the eutherian ovum. 
EXPLANATIONS OF THE REFERENCE LETTERS USED IN THE FIGURES. 
or continuation of body cavity, dr decidua reflexa of uterus, Æ embryo, e epiblast, 
— Fohn A. Ryder. 
DECEMBER 31, 1885. 
PHYSIOLOGY. 
THE EXISTENCE OF TWO KINDS OF SENSIBILITY TOWARD LIGHT. 
* 
