1879.] 203 [Minot. 
dicular to the surface of the layer, of which the cell forms a part. 
In the primitive epitheliums of the embryo these three axes are 
nearly equal in length, hence the tissue is said to be composed of 
cubical cells. That these characteristics are true of all classes can- 
not, I think, be questioned by any one who will consult those em- 
bryological memoirs in which accurate histological figures of the 
early stages are given. That the germinal layers have the charac- 
teristics described I can affirm from direct observations of my own 
upon Toxopneustes, Doris, Lymnaeus, Ancula, Clepsina, Leptoplana. 
Palaemon, etc., and certain vertebrates. These early features are 
sooner or later in part obliterated. 
2. Very early in the course of development the ectodermic cells be- 
come smaller amd multiply faster than the cells of the entoderm. This 
fact, which I have pointed out in a previous publication! seems to 
have hitherto escaped notice. It appears to me to have great im- 
portance, for it indicates a definite and fundamental difference be- 
tween the two germinal layers. The large size of the ectodermic 
cells is accompanied and perhaps caused by the development of yolk 
granules, which apparently are almost if not quite confined to the 
entoderm. 
In my article just cited, ] have referred to certain observations 
that do not accord with the generalisation made. To those observa- 
tions I am still inclined to attach little importance. Since my first 
paper appeared, a most important exception has been made known. 
In the sponge embryo, at the time it assumes the churacteristic egg 
shape, there are, as is well known, two distinct parts, which apparently 
correspond perfectly to the two first layers of the diaderm? of the 
remaining Metazoa,— one half of the larva is composed of compar- 
atively small ciliated cells, apparently the analogue of the ectoderm 
in other larve, and one half of very large cells, charged with nutri- 
tive granules, having in short the characteristic features of ento- 
dermal cells. F. E. Schulze and Mecznikow both agreed in re- 
garding the large celled layer as entoderm, the other as ectoderm. 
1 Proc. Boston Soc. Nat. Hist., xix, 165, (1877). 
2 The term diaderm is used to designate that stage in which the two layers ento- 
derm and ectoderm are clearly differentiated. This designation is preferable to 
any other I know, because it indicates the essential feature, the presence of two 
distinct layers, that feature which forms the basis of animal morphology. The ex- 
istence of the diaderm is an established fact, while the existence of the Gastrula is 
at least doubtful in some cases. It must be remembered also that the formation of 
two layers often precedes the formation of the gastrula. 
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