'^joum'lIlISiry '^^ 807716 Gvouj^s of Orustacea. 45 
III. Free Copepoda. 
We have studied the formation of the blastoderm in several 
marine Copepoda; the ovum which is constituted like that of other 
Crustacea, undergoes— as Claus has observed — a total segmentation 
of the vitellus. The form and disposition of the segments at the end 
of the process are the same as in Chondracanthus and the Amphi- 
poda; and the formation of the blastoderm results from the same 
series of phenomena. AVe have here made some very interesting 
observations, which have convinced us that the germinal vesicle 
does not disappear, but divides to form the nuclei of the segmenta- 
tion-globes. 
Conclusions. 
The blastoderm in Crustacea may be formed : — - 
(1.) After total segmentation of the vitellus [Chondracanthus ; 
Gammarus locusta ; Copej)oda'\. This phenomenon may present 
many varying characters, but it invariably occurs in such a way 
that the globes have ultimately a pyramidal form, and that each 
furnishes a cell to the blastoderm. The formation of these cells 
always results from the multiplication by division of the ovum- 
cells, and the separation in each of the globes of the two chief 
constituents of the vitellus. 
(2.) After partial segmentation (Decapoda — Kathke). 
(3.) Without segmentation, according to the process pointed 
out by La Valette St. George in Gammarus pulex, and which we 
have demonstrated in several fresh-water species of Grammarus. 
(4.) Without segmentation, according to the very different pro- 
cess which we have found in various Lerneae. 
These different processes are in the main perfectly identical. 
The mode of formation of the hlastoderm, varying, as it does, 
within the limits of the same natural groups, and likewise within 
the limits of the same genus {Gammarus), cannot in any way serve 
as a basis of classification ; and we may say the same as to 
segmentation which takes place in an identical manner in very 
distinct and unallied groups (Batrachia, Gammarus locusta), and 
varies exceedingly within the limits of the same group {Chondra- 
canthus, Gammarus locusta). 
