^Journal. S^iS^ PROGRESS OF MICROSCOPICAL SCIENCE. 115 
of bichromate of potass, it will be seen that the blastoderm does 
not exhibit itself over the whole surface of the egg, but that it appears 
in islets. On account of the deep colouration of the vitellus it is 
impossible to view this phenomenon satisfactorily without destroying 
the ovum. After the blastoderm has extended around the yolk, the 
embryonic membrane detaches itself from it. It is this membrane 
which you have regarded as formed by the division of the primitive 
envelope of the ovum, and which Claparede has designated under the 
name of deutovum. I consider it to be the homologue of the larval 
membrane [Larvenhaut] of Crustacea, and this is evidently the homo- 
logue of the amnios of insects. I hope to demonstrate this more fully 
in a memoir on the amnios of Arthropoda which I hope to publish 
soon. It is in a short time after the formation of this embryonic mem- 
brane that we see appear between it and the blastoderm the first 
amoeboid cells that Claparede thought arose at a later period. When 
in my work I designated these amoeboid corpuscles under the name of 
blood-globules, I desired merely to indicate their relation to the cells 
of the blastoderm which at the date of the appearance of these 
hcem-amoehce are the only cellular formations of the ovum. It seems 
to me that we shall have to admit two modes of origin of blood-cor- 
puscles. In certain ova that I have watched through their entire 
period I have never seen any of these amoeboid cells penetrate into 
the interior of the ovary. Claparede asks if you have not taken the 
parasites of Anodon for those of Unio, or, rather, if they are not the 
same animals which live in Belgium on Anodon, and in Geneva on 
Unio. In commencing my researches I placed a hundred Anodons 
collected from the neighbouring pools in a locality with running 
water, where I subsequently placed the Unio hatavus. Four weeks 
after the two had been placed side by side, I found — as I have since 
frequently done — the parasite of U7iio on Anodon, and vice versa. It is 
hardly possible to confound the two parasites, as those of Unio have 
five suckers on each side of the ventral orifice, and those of Anodon 
have from thirty to forty." Such being the substance of M. Bessels' 
letter, the following is that of M. Van Beneden's reply : — " The 
disagreement which the writer points out has no real existence, since I 
declared distinctly that I had examined only the ovarian eggs. What 
I said was, ' the germinal vesicle is relatively larger as the egg is 
younger.' I am astonished that MM. Bessels and Claparede did not 
perceive that I spoke of the composition of the ovum before deposir 
tion. As to the Atax, which M. Claparede supposes to proceed from Unio 
rather than Anodon — which I shall send him alive if he wishes — all 
those used in my researches were taken from the same locality in the 
neighbourhood of Louvain, and where I have never seen any but 
Anodon. For the rest, M. Bessels' observations show that the same 
Atax may live on both mollusks. I would remark, in conclusion, that 
the amoeboid cells which appear around the blastoderm seem to be 
perfectly analogous to those pretended parasites which appear in the 
same conditions on different mollusks, and which Nordmann* has 
named Cosmella hydrachnoides in Tergipes Edwardsii." 
* 'Ann. des Sci.,' 1846, vol. v., 3"'*' Serie. 
