44 Formation of the Blastoderm Ho^S^iLuiry^^^^^ 
round the germinal vesicle, forming with it the first embryonic 
cell, while the nutritive elements pass towards the periphery. 
Then the first embryonic cell divides, at first into two and then 
into four, and these few large cells travel towards the periphery, 
passing through the larger of nutritive materials. On reaching 
the surface these cells continue to divide, and thus form a cellular 
layer which extends more and more, to form finally the blastodermic 
membrane. 
This mode of formation of the blastoderm is essentially iden- 
tical with that we have described in Chondracanthus. Here also 
the formation of the first cellular layer of the embryo results from 
the separation of the nutritive and protoplasmic elements of the 
yolk, and from the multiplication by division of the ovum-cell. The 
only difierence is that in Chondracanthus this separation takes 
place after the division of the cell, whilst in Caligus, Anchorella, 
&c., it is the first phenomenon manifested by the fecund ovum. 
II. Amphipods. 
What we have said regarding the mode of formation and the 
constitution of the ovum in Chondracanthus, may also be said of 
the ovum of Amphipods. The ovum of Amphipods does not pre- 
sent a vitelline membrane ; the membrane which Meissner took to 
be such is a production of the cells of the blastoderm, and conse- 
quently an embryonic membrane. The orifice of such a membrane, 
then, cannot be called a micropyle, as Meissner has termed it. 
We have most carefully investigated the process of total seg- 
mentation of the yolk in Gammarus locusta, and many other 
marine species. It does not exhibit the peculiar features that we 
pointed out in Chondracanthus; but the diagrammatic figures 
which we have given in our memoir are identical with those given 
by Ecker in his ' Icones Physiologicse,' of the segmentation in the 
egg of the frog. When segmentation is completed, the ovum is 
found to be composed of a great number of pyramidal segments, 
and then there occurs abruptly , a separation between the nutritive 
elements and the protoplasmic matter of the several segments. The 
protoplasm, enclosing the nuclei of the globes, passes to the peri- 
phery of the ovum, to form there the cells of the blastoderm. 
These become fused together, and form a continuous cellular, which 
gives rise by secretion to a primary cuticular membrane. This it is 
which was styled the vitelline membrane, and which presents the 
pretended micropyle. 
We have demonstrated, in the case of Gammarus jr>ulex and 
other fresh-water species of Gammarus, the correctness of M. 
Yalette's observations — the blastoderm is formed without any pre- 
vious segmentation. 
