42 Formation of the Blastoderm TThe Monthly Microscopical 
L Journal, January 1, 1869. 
strated positively in Nicothoe by P. J. Van Beneden (1848) ; in 
Phyllopoda of the genera Artemia and BrancMpus, by Leydig 
(1851) ; and finally by Clans in various Copepoda. Then natu- 
ra''ists admitted that the vitelline globes situate at the periphery of 
the ovum underwent after the segmentation a process of " clearing 
up," to form the cells of the blastoderm, and that the central 
globes became fused together to form the mass of nutritive sab- 
stances surrounded by the blastodermic vesicle. 
In other Crustacea the blastoderm is formed without complete 
segmentation. The Decapoda exhibit, according to Eathke, only a 
partial segmentation of the vitellus ; but the illustrious embryologist 
did not know of the relation which exists between these segmen- 
tation-globes and the blastodermic cells. If we may believe M. 
Dohrn, the blastoderm in Asellus is formed from a blastema in the 
same way as was pointed out by Zaddach in the Phrijganidse and by 
Weissmann in dipterous insects. M. de la Yalette St. George has 
observed in Gammarus pulex that there occurs at the commence- 
ment of development a separation into two parts of the elements of 
the vitellus. The vitellus of formation Lecomes divided into minute 
masses, each enclosing a nucleus, and perhaps represent portions of 
the germinal vesicle. These small nucleated masses are so many 
cells which have only to pass to the circumference in order to form 
the blastodermic membrane. 
We have made a special study of this important histogenetic 
question, and our researches have been principally made upon these 
groups of Crustacea, Lernese, Amphipoda, and Copepoda. 
I. Lernead^. 
Two very distinct types of development are to be found in this 
singular group of animals. The first is presented to us in Chon- 
dracanthus ; the second in Galigus, Anchorella, ClaveUa, Congeri- 
cola, Lernea, and Eudadylina. 
\§t Type. Cliondr acanthus. 
In order to give an intelligible account of the first embryonic 
phenomena, it is necessary to say a few words on the mode of 
formation and the constitution of the ovum. The ovum in Chon- 
dracanthus is at first a minute protoplasmic cell, without a mem- 
branous wall, exhibiting very active amseboid movements, and 
under certain conditions capable of absorbing solid particles and 
granules of carmine. This cell is at first clear and transparent, 
and becomes charged gradually with highly refractive nutritive 
matters, and soon attains its normal volume. In passing along the 
oviduct the ovum becomes surrounded by a membranous envelope, 
which is a product of the secretion of the wall of the oviduct^ and is 
