Miscellaneous. 307 
Tn animals of which the vitcllus undergoes segmentation after the 
extrusion of the ova, it is at from four to six hours after the latter 
that the polar globules begin to appear, that is to say, at from twelve 
to twenty-four hours after the disappearance of the germinal vesicle. 
Their production takes from two hours to three hours and a half; and 
about two hours after their completion the segmentation commences. 
The mode of production of the polar globules is remarkable : it is 
a true gemmation from the limpid substance of the vitellus, followed 
by a constriction and then by the transverse division of the base of 
the process. It commences by the retreat of the granules of the 
vitellus from a circular portion of the surface about 0°05 mill. in 
diameter, leaving the hyaline substance quite clear. In a few minutes 
this transparent portion forms a hemispherical projection, which 
afterwards becomes conical. Its base contracts, giving it for a mo- 
ment the form of a cylinder about 0°02 mill. in diameter, and twice 
as long; the contraction soon causes a constriction, the projection 
becomes pyriform, and it rapidly separates from the vitellus, with 
which, however, it remains in contact. These phenomena last from 
fifteen to twenty-five minutes, during which the vitellus is the seat 
of slow deformations, but it resumes its spherical form immediately. 
The phenomena are reproduced a second time in the Mollusca, and 
even a third and fourth time in most other animals. In those in 
which the vitellus does not entirely fill the vitelline membrane, as in 
Nephelis, there are eggs in which the process of the hyaline sub- 
stance forms at first a long cylinder, which contracts and becomes 
segmented transversely in three places, forming three polar globules. 
The globules have no distinct walls or cavities ; and the few vitelline 
granules which pass into their interior show no trace of Brownian 
movements. 
After the completion of the last of these polar globules, or even 
before this, a very curious phenomenon takes place, characterized by 
the successive union of two or four globules into a single one, which 
persists up to the period of exclusion, and in which a cavity distinct 
from the wall in some species, or from one to three nuclei, or some- 
times some granules which did not exist originally make their ap- 
pearance. This union is accomplished in two modes: sometimes the 
first-formed globule gradually diminishes in size, its substance pass- 
ing into the next one until it completely disappears ; and when there 
are more than two globules, the phenomena is repeated until only 
one remains in contact with the vitellus; in other cases the globules 
mutually coalesce by the gradual enlargement of their plane of con- 
tact. A few moments after the reduction of the polar globules to a 
single one, segmentation commences; sometimes, however, it is set 
up a little earlier. 
The Moilusca present a peculiarity in this respect. At the mo- 
ment of the commencement of segmentation, after the union of the 
polar globules, another and somewhat larger globule makes its ap- 
pearance, which elevates the preceding one, and strongly refracts 
light. This second globule rises perfectly formed from the midst of the 
superficial substance of the vitellus without any preliminary clear space. 
It at the same time elevates, at the surface of the vitellus, a very 
