REVIEWS OF RECENT LITERATURE. 
ZOOLOGY. 
Hammar’s Layer. — Professor Hammar had previously? given 
facts that led him to believe that blastomeres of many animals are 
connected by a continuous membrane-like expanse of protoplasm 
that forms the outermost part of each cell and passes over from cell 
to cell. This cleavage would result in only partial subdivision and 
not in complete isolation of cells. Moreover, the cleavage cavity 
would be a hollowed-out interior space, not outside space extending 
in between the cells. j 
In a recent paper? he advances some new facts and considerations 
in support of the thesis that this connection of blastomeres is a 
protoplasmic part of the egg and not a dead “membrane,” and, 
secondly, that it is a primary connection always there, and not a 
secondary connection established from time to time as are the filose 
threads previously described by others and now, apparently, seen 
by Hammar in preserved eggs. HÀ A. 
What holds Blastomeres together ?— Curt Herbst? adds to his 
previous work upon the chemical environment of echinoderm eggs 
and larva a series of observations upon eggs in an artificial sea- 
water free from calcium. . 
Eggs in various stages of cleavage thrown into such a solution 
may continue to cleave and even to differentiate so that some cells 
may show cilia; but the blastomeres do not cohere, but fall apart 
and develop separately. This, however, takes place only when the 
eggs have been shaken to remove the membrane, that else holds the 
cells together to some extent. 
‘When the isolated cells are put back into water with lime they 
cleave and cohere, so that many little larvae result from one egg. 
The cause of the falling apart of the blastomeres is not clear. 
Herbst supposes they are normally held together by the tension of 
1 See Review, American Naturalist, 1896, p. 597; and 1897, p. 454- 
z Arch. f. mikr. Anat., Bd. lv. February, 1900. 
3 Arch. f. Entm., Bd. ix, Feb. 20, 1900. 
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