400 On the so-called Biogen Liquid. 
Without further prefatory remarks, I proceed directly to take 
up one by one the facts, the theory, and the speculations. 
I. The primitive egg has been made the subject of much re- 
search by Prof. Agassiz, especially in the department of Mollusca. 
Ascending even beyond the first existence of the egg, he has 
shown us the ovary itself in the process of development. This is 
composed of sacks or pouches varying in form and size, in which 
the eggs are formed. The sacks are filled with an homogeneous 
and transparent liquid. Soon this liquid becomes granular, that 
is, consists of cells, and the cells becoming more and more nu- 
merous, give birth to a little opaque sphere, which is the vitellus. 
The germinative vesicle and the germinative spot have appeare 
during the formation of the yolk, and sometimes even prior to 
this period ; but at the moment when the phase of division com- 
mences, both the spot and the vesicle generally disappear, and in 
the interior of each of the spheres produced by the division, is 
seen a clear space. 
A Résumé of these observations has been published.* Since 
then, Mr. Desor has observed analogous phases in other animals. 
He published them, as he had a right to do, but he should have at 
least declared at the outset, that he was doing nothing more than. 
repeating the observations which another had made before him. 
II. When an egg has reached that point of its history which 
is called its maturity, it is distinguished by the following charac- 
ters :—a spherical mass, more or less opaque, which is the yolk ; 
in the center of this is found a much smaller sphere, the germina- 
tive vesicle, containing another substance, usually transparent ; 
then in the interior of this last, a sphere or spheres still smaller, 
the germinative spot or spots. . 
The epoch of the appearance of the germinative spot varies as 
it would seem within very considerable limits. This is not the 
place to discuss this question. Let it be observed, however, that 
they exist in every egg when it is mature, and that they disappear 
from every egg when it enters upon the period of division. 
But among the Nemertes a curious phenomenon is observed. 
Generally as we have just said, the vesicle and spot disappear be- 
fore the division of the yolk, or, at least, at the moment when it 
divides in halves. The secondary spheres resulting from the sub- 
divisions of the vitelline mass, have then, each one in its interior, 
a clear space. The question has been raised, what part the ger- 
minative vesicle plays in the history of the eggs? Is the division 
the consequence of its disappearance? in other words, is its con- 
tent necessary to effect the division. The germinative vesicle 
has been considered as containing the primitive elements of the 
* Lectures on Comparative Embryology, by Louis Agassiz. Boston. January, 1849. 
