f 
ik ~ 
2 - Chango of shape: the cell becomes s nary c or awite spheric 
1880.] A Sketch of Comparative Embryology. 99 
The various envelopes which eggs ever have, may be classed 
under four categories: First, a very thin and delicate one, the 
proper membrane of the cell itself, which ought always to be dis- 
tinguished as the vitelline membrane; second, the ovarian envelope 
which is secreted around the egg-cell by the tissues of the ovary ; 
third, the envelopes secreted by the oviduct, which may form an 
envelope of nutritive matter, or a protective shell, or both, as in 
the hen's egg, of which the nutritive white is secreted by the 
upper part, the calcareous shell by the middle part of the oviduct; 
Jourth, coverings secreted by accessory glands, such as the slime 
in which the eggs of snails are imbedded, or the shells in which 
leeches lay their eggs. By adhering to this classification the 
student will be able to follow with profit the labyrinth of special 
description., To enter into further details would lead aside from 
the object of this article: let it suffice to have pointed out the 
possibility of manifold variations, and to have emphasized the 
fact that the egg-cell is the important and only essential part of 
an egg. 
The egg-cell always arises from a germ-mass, called Killapi 
in German. The germ-mass is at first composed of cells all 
essentially identical in microscopic appearance. Single isolated 
cells then transform themselves into eggs, while their surrounding 
fellow-cells play the rôles of nurses and purveyors. To avoid 
inaccuracy it must be added that in some cases the germ-mass 
does not consist of distinct cells, but contains numerous nuclei 
which ultimately become the centers of distinct cells; but before 
this separation the differentiation of the ova begins. In both — 
methods of development some cells enlarge to form the eggs, 
others supply the enlarging and growing cells with nutritive 
material. It is impossible to enter upon this sybject further than 
to say that the form and disposition of nutritive cells varies 
extremely in different animals, while the changes in the egg are 
much more uniform, so that it is possible to describe in arene 
terms the development of the ovum. aS 
The Moadations which occur in the growing ——— are as cs 
follo Ows ee 
= Change of size: the cell enlarges, it being a rule, no exception S 
to which is, I believe, known, that the mature egg-cell oe 
: erry larger than any of the other cells in the ASA of the 
