AO4. REPORT—1863. 
within them a few minute granules which have been rendered obvious by the 
action of the iodine. 
The vitellus is entirely composed of minute’ spherical corpuscles of appa- 
rently homogeneous structure, about 0-0002 of an inch in diameter, along 
with granules so small as not to admit of measurement. There is no obvious 
vitellary membrane in the mature ovum, but I have satisfied myself of its 
presence while the ovum is still in a very young state. In other species, 
Hydractinia echinata for example, this membrane is very obvious in the 
ovum just before segmentation. There is no trace of a micropyle in the ovum 
of this or of any other hydroid which I have examined. 
There is never more than a single ovum in each gonophore of Laomedea 
flecwosa; and as this ovum continues to enlarge, it presses back the spadix 
until the latter is reduced to a small hollow projection in the bottom of the 
gonophore. 
Up to this time the germinal vesicle continued quite distinct, but it now 
entirely disappears (fig. 20 B). The disappearance of the germinal vesicle is 
unaccompanied by any apparent change in the structure of the ovum, which 
retains the same peculiar composition of spherical corpuscles and granules. 
I have no doubt that the vesicle now ceases to exist, and that its disappearance 
is not due to its being merely concealed in the mass of the vitellus. It has 
probably burst, and in so doing must have liberated its peculiar contents, 
which will then of course be no longer visible in the vitellus. The disap- 
pearance of the germinal vesicle is probably the immediate result of impreg- 
nation ; for I have seen active spermatozoa about this time in the cavity of 
the female gonophore. 
It is useless to speculate upon the influence which the liberated contents 
of the germinal vesicle may exert in exciting the new series of phenomena 
which are now about to take place in the ovum; at all events, shortly after 
the disappearance of the germinal vesicle, the process of segmentation sets in. 
This process is certainly not preceded by the visible appearance of a new 
nucleus destined to take the place of the germinal vesicle. Itis quite possible 
however that such a nucleus may exist, though, from its small size and from 
being so deeply imbedded in the mass of the vitellus, it may have eluded our 
attempts to discover it. 
The first step observable in the segmentation-process is the cleavage of the 
yolk into two segments (fig.20 C), immediately followed by the cleavage of these 
into other two, so that the vitellus is now composed of four cleavage-spheres 
(fig. 20D). In none of them, however, can a nucleus be as yet demonstrated. 
The segmentation would now appear to proceed very rapidly, but, perhaps, 
not always with absolute regularity ; for it would seem occasionally to advance 
more rapidly in some of the previously formed spheres than in others. By 
the time that the vitellus presents about thirty-six or more cleavage-spheres 
(fig. 20 E) we begin to recognize in some of these spheres a distinct nucleus, 
while as the spheres become smaller and more numerous the nuclei become 
more and more apparent, until at last there may be seen in every minute sphere, 
of which the segmented yolk is composed, a brilliant nucleus, visible not only 
in the superficial spheres, but also in the deeper ones which come into view 
when the ovum is broken down under the compressor (fig. 20 FandG). It is 
therefore highly probable that in the earlier stages also a nucleus exists in every 
cleavage-sphere, but that in consequence of the thickness and opacity of the — 
enveloping vitellus it is withdrawn from observation. The cleavage-spheres at 
this stage present the same peculiar structure which we find in the yolk just 
before the commencement of segmentation, consisting as they do of minute 
