24 ~—-Dr. W. Salensky on Héickel’s Gastrea Theory. 
Trochus, Vermetus, Entoconcha, &c.) takes place in the same 
manner. 
The further developmental phenomena of the animals which 
pass through the Blastula-stage in the course of their ontogeny, 
may occur in different ways. If we commence with the 
embryonal state of the Ascidia, which constitutes a flattened 
vesicle (Pl. V. fig. 4), and in which the differentiation into 
two germ-lamellz has already been effected, we see that the 
subsequent phenomena consist in the whole embryo acquiring 
a cup-like form (fig. 5). This cup, consisting of two layers, 
afterwards passes into the Gastrula-stage (as 1s well known in 
the Ascidia, Amphioxus, Lumbricus, &c.). In consequence 
of these changes (of the invagination) the stomachal cavity 
of the Gastrula is produced ; but the stomach-wall has been 
differentiated earlier, during the flattening. 
Whilst in the last-mentioned cases the embryo (Diblastula) 
is converted into the Gastrula-form, the corresponding Diblas- 
tula-form of the insect undergoes quite different changes. In 
these the entoderm sinks into the nutritive vitellus, and is 
gradually covered from without by the exoderm. The diver- 
gence of the two corresponding stages of development in the 
Ascidia and in Hydrophilus, both of which may be derived 
from a common Dzb/astula-form, is elucidated by the two 
figures 6 and 7, in Plate V.* 
These differences in development lead finally to the totally 
divergent conditions of the subsequent embryonal phenomena 
in these two animals. Whilst in the Gastrula (Ascidia) the 
intestinal cavity is already sketched out, it will only be formed 
afterwards in the insect, and, indeed, in quite another 
manner than in the Gastrula. 
From this it is clear that the formation of the stomachal 
eavity in these two cases is a secondary phenomenon, 
governed by different later conditions of the exodermal 
and entodermal layers. The most important phenomenon 
in both cases is the differentiation of the germ-lamelle from 
an indifferent cell-layer, therefore that stage of development 
represented in figs. 4 and 5. They are of great importance, 
chiefly because they represent the first processes which are 
common to the two forms (Ascidia and Insecta), and from 
which the divergence of the subsequent developmental forms. 
starts. 
* The developmental states which occur in Hydrophilus at the period 
of the closing of the groove (see Kowalevsky, oc. cit. Taf. ix. figs. 21-25) 
may serve as an inducement for assuming the occurrence of the Gastrula- 
stage in this animal. But to me this assumption seems to be scarcely 
justified, because the same process takes place without any such formal 
condition in Gastropacha pim. (See Kowalevsky, Taf. xii. figs. 1-6.) 
