18 Dr. W. Salensky on Hiickel’s Gastreea Theory. 
tirely disappears) ; the inner one is developed into a so-called 
six-hooked embryo, which consists only of homogeneous cells. 
We may certainly compare with a Planula that state of the 
embryos of the Zwnie and Bothriocephalide in which they 
consist of a two-layered body (therefore before the development 
of the embryo and the embryonal membrane). 
In the other animals which pass through the Morula-stage 
the differentiation of the germ-lamelle takes place in an exactly 
similar manner asin the above-mentioned cases (some Copepoda, 
some Gammaride, probably the Ctenophora and the Ccelen- 
terata, Hydroid polypes, and Sponges). After the segmentation 
the uniform cells divide into two layers, which represent two 
germ-lamelle, and become further developed into the organs. 
Unfortunately, in the investigations of the development of 
many of these animals, the question of the formation of the 
germ-lamelle has been very little referred to. It appears to 
me that in many instances the entoderm has been explained 
as the nutritive vitellus. But until the formation of the intes- 
tinal epithelium in the lower Crustacea has been further in- 
vestigated, we may affirm with perfect justice, from the analogy 
of the developmental processes in animals which have been 
better investigated, that the central spherules, abounding in 
fat, of the crustacean embryos really form the entoderm and 
not the nutritive vitellus. ‘That in many instances we can see 
no cells in this part is due to its opacity. In Astacus fluvia- 
tilis the peripheral parts of the cells of the entoderm, from 
which the intestinal epithelium is formed, are also very diffi- 
cult to observe, and only become distinct when they are tinged 
with carmine or some other colouring-material. At any rate 
in this instance also we obtain, as the result of differentiation, 
the same temporary body-form, consisting of two layers, 
and possessing no cavity in its interior—that is to say, the 
Planula. 
In some instances, in which we decidedly have the same 
process before us, it may be obscured by certain subsidiary 
phenomena. In most cases this masking is caused by the 
occurrence of the nutritive vitellus, which is accumulated in the 
ege in larger or smaller quantities. Such cases occur, for ex- 
ample, in the Cephalopoda, in Reptiles and Birds, and also in 
Fishes. Here the egg-cell which becomes segmented is 
situated at one pole of the egg. The segmentation may be 
compared to the regular segmentation, inasmuch as the cells 
produced by the segmentation are at first uniform and subse- 
quently differ from one another. It is only at a later period 
that the differentiation of the germ-lamelle occurs in this 
ageregation of cells ; the germ-lamelle are mutually arranged 
