OF PLANOCERA ELLIPTIC A. 
13 
For a short time after the eggs have been laid we still observe a clear space towards 
one point of their surface, appearing however quite small on account of the increase 
of the vitelline substance, now filling up densely the whole sphere (fig. 13). 
The absolute size of the eggs varies within certain limits; a few being found one 
third smaller than the usual size. The form itself is subjected to variations ; generally 
circular, it sometimes elongates and becomes oblong or oval in some few. 
V. 
THE DIVISION OF THE VITELLUS. 
The division of the vitellus or yolk commences about twelve hours after the depo- 
sition of the eggs, and last about twenty four hours. This phenomenon is subjected 
to a mathematical law as far as it affects the substance, but as there is already present 
a vital agent, the living principle of the forthcoming being, an allowance is made for 
variations. Let us consider the subject under both points of view. 
Generally the primitive yolk’s sphere divides into two spheres of equal size (figs. 
23, 24 and 25), then these two are subdivided, making four spheres (figs. 34 and 35), 
then eight spheres appear (figs. 38 and 39), then sixteen (fig. 43), and so on until the 
yolk’s mass is so much subdivided that it presents the appearance of a mulberry, the 
mulberry-shape or last stage of the division of the vitellus. Such is the physical or 
mathematical law which governs the matter, as it will strike every one when observing 
the phenomenon of the division. 
Now the variations. While the foregoing is taking place there are eggs in which 
the process of the division appears rather fanciful, as if the living principle was already 
assuming a marked preponderance over the matter itself, and subduing the latter to the 
power of organism. Thus in fig. 15 one point of the sphere is depressed : in fig. 16 
the depression has become a groove. The sphere sometimes is split open as in fig. 
1 7. Sometimes, again, a groove on one side and a slight depression on the other side 
are observed (fig. 18). Still others show two symmetrical grooves opposite each other 
and connected by a transverse line (figs. 19 and 20), which foreshadows a division 
of the mass into two halves, equal or unequal, symmetrical or asymmetrical, though 
never circular, as represented in figs. 21 and 22. 
In October 1847, Dr. A. A. Gould observed in the eggs of Eolis gymnota , from the 
Boston harbor, cases in which the yolk was divided into three spheres. This fact was so 
new, and I may say, so unexpected, that it struck every one then present and interested 
in embryological researches, as indicating a new law or principle hitherto unnoticed 
In the division of the yolk. Now, a similar case may be seen here (fig. 29): three 
perfectly distinct spheres, of unequal size, it is true, but three spheres only. Figs. 27 
