338 Miscellaneous. 
delicate translucid pellicle, of mucous appearance, beneath which all 
the phenomena of segmentation take place. In unfecundated ova 
the second polar globule is not produced, any more than the thin 
pellicle which it elevates; in these no phenomena of segmentation 
take place in the fresh-water Mollusca, and in most of the marine 
species. The polar globules produced by gemmation appear, but 
they remain distinct and contiguous. 
Hitherto observation has led to the supposition that the first 
cells of the embryo are always formed by the segmentation of the 
vitellus; but the author has found that there are certain animals 
in which the segmentation of the vitellus does not occur, although 
their fecundated ovum presents a very distinct blastoderm. The 
production of the blastodermic cells, which marks the close of seg- 
mentation in other cases, takes place here by a process of gemmation. 
This phenomenon is observed most distinctly in the eggs of the 
Culiciform Tipulidee, in which the whole of the blastodermic cells, 
and not merely the polar globules, are produced by gemmation. The 
same thing probably takes place in the other Tipulidee, but the im- 
perfect transparency of the envelopes of their eggs renders observa- 
tion difficult. The arrangement of the blastoderm in the Muscidee 
leads to the supposition that it is formed in the same manner; and 
the author thinks it possible that similar phenomena may occur 
throughout the Diptera. 
These phenomena are described as follows by M. Robin :—The 
vitellus, which up to the deposition of the egg completely filled the 
vitelline membrane, is retracted, and immediately afterwards the 
production of the polar globules commences towards the smaller ex- 
tremity of the egg. These are produced as above described, and 
frequently two are formed at the same time close together. They do 
not, however, coalesce to form a single globule, but, on the contrary, 
increase in number by scission. At the same time, a nucleus is gra- 
dually developed in them, giving them the character of true cells. 
Lastly, instead of resting on the side of the embryo almost like a 
foreign body, they soon take part in the formation of the blastoderm. 
In the Culiciform Tipulide the cells forming the blastoderm are 
produced quite independently of any segmentation of the vitellus. 
They begin to show themselves at the extremity of the vitellus oppo- 
site to that at which the polar globules are produced. Coming into 
existence side by side, they gradually occupy the rest of the surface 
of the vitellus. It is at the expense of the hyaline substance of the 
vitellus that the blastodermic cells are produced ; and their formation 
is by a process of gemmation. The surface of the vitellus, observed 
by transmitted light, presents small hemispherical projections, the 
elevation of which gradually increases. When their height equals 
their breadth (which is from 0°0014 to 0-0016 mill.), they begin to 
be mutually compressed. At an hour or an hour and a quarter after 
this commencement, the adherent extremity of each projection is’ 
gradually constricted, so rapidly as in a few minutes to separate from 
the vitelline substance with which it was continuous. Thus are 
produced as many cells as there were projections, forming a single 
series of colourless, transparent, blastodermic cells surrounding the 
