668 ORIGIN AND TRANSFORMATION OF ANIMALS. 
The marsupial animals, such as the kangaroo, forming no 
real exception, as the seclusion of their young in the pouch 
is only a second act of gestation. In other species—all of 
them oviparous—the offspring, at the moment of leaving the 
egg, differs completely from both its father and its mother. 
It may possess organs which they have not, and be destitute 
of organs with which they are furnished, so that changes and 
metamorphoses are required to bring it back to the original 
type. M. Quatrefages proposes to restrict the term trans¬ 
formation to the designation of those changes which the germ 
experiences in becoming an embryo, or which it undergoes 
while still enclosed in the egg. Metamorphosis , in like manner, 
designates changes altering the character of the creature, 
and, occurring after it has left the egg, or been hatched. 
Geneagenesis refers to the changes which “ affect the genera¬ 
tions themselves.” 
In discussing the transformations of the egg, M. Quatre- 
fages refers particularly to his own observations of the Serpula 
and Teredo. After the laying of their ova, the eggs, whether 
fecundated or not, exhibit an internal commotion; “a mys¬ 
terious force agitates the yolk; granulations accumulate now 
at one point and now at another,” so that the shadowy mass 
changes its aspect every moment. M. Quatrefages considers 
that similar changes take place in the eggs of higher animals 
although they may be slower, and more difficult to trace. 
In the Serpula and Teredo eggs the agitation causes the 
“Purkinje vesicle” and the “spot of Wagner” to disappear. 
If the eggs have not been fecundated, the movements be¬ 
come accelerated and irregular, and, finally, decomposition 
ensues. All through the animal kingdom the male element 
appears to excite and regulate the germinating force. In the 
eggs of the creatures named, a little nipple appears on 
the surface of the altered yolk, from which one or two 
transparent globules are expelled, the use of which is 
unknown. 
This occurs whether the eggs have been fertilized or not. 
If fertilized, the expulsion of the globules is succeeded, 
“ whether it be in the mammalia or the serpulae,” by a short 
period of repose. When activity recommences constrictions 
become visible, and the yolk assumes a mulberry aspect. 
The details of the process vary in different animals, “but in 
all, the consequence of the phenomenon is the formation of a 
primitive organized layer which envelopes the yolk, and is 
called the blastoderm. As soon as organization begins, it 
assumes distinctive characters; “ the germ becomes the 
embryo, and from its origin reveals the fundamental charac- 
