98 A Sketch of Comparative Embryology. (February, 
the Infusoria have strengthened the movement of return towards 
the earlier doctrine, which had been for a while crowded aside by 
the over-hasty advocacy of the protoplasm theory. Bütschli es- 
pecially has made it extremely probable that all Infusoria are but 
highly specialized and curiously modified unicellular beings. 
It is certainly safe to assume for the present that no life can ex- 
ist outside of cells,and that all the phenomena of development 
must be reduced to terms of cell-life. The first point, therefore, 
to be settled is the relation of the sexual products to the cells 
from which they are derived, and the multiplication of which they 
effect. I shall give an hypothesis of these relations, which I 
have formed, and which is the only one, so far as I am aware, yet 
proposed. Whether this hypothesis will ultimately prove correct 
or not, it is impossible to foresee. As it still appears to me plausi- 
ble, I shall venture to reproduce it here. To explain it, it is 
necessary to premise brief accounts of the structure of the sexual 
products (genoblasts) and their development. We will begin with 
the egg. 
The essential part of every egg is developed from a single cell, 
which undergoes certain modifications, probably nearly the same 
in all animals, thereby acquiring the definite characteristics which 
distinguish it as an egg-ce// from an ordinary cell, and from all 
other specialized forms of cells. 
The eggs of different classes and even species of animals are. 
as is well known, extremely unlike in appearance. The dissimi- 
larity refers chiefly to size, and to the nature and number of mem- 
branes or envelopes by which the egg-cell proper is surrounded 
by the parent. Thus in the hen’s egg, the yolk alone represents 
the part formed by the egg cell, while the white of the egg and the 
egg-shell are only secondary envelopes, the former serving to 
nourish, the latter to protect the yolk, which is the essential pa 
the true egg. 
Now, it is well known that mere size does not enter into the 
determination of the real affinities of animals and plants. The 
smallness of the rat does not show that it is related to the frog 
rather than to the elephant, and from our present point of view the 
size of eggs is meaningless. The egg-cells are large in all birds 
and reptiles, in the sharks, rays, ganoids and Cephalopoda, small 
_in mammals, bony fishes and nearly all invertebrates, inarpediate 
iñ —_ 
