20 Dr. W. Salensky oa Hackel’s Gastrexa Theory. 
secondary phenomenon, as we shall see hereafter. The stage 
in which the first differentiation of the germ-lamelle occurs is 
of great importance, because it may serve for comparison with 
the corresponding stages in other animals which also pass 
through a Blastula-stage in their ontogeny. By the occurrence 
of the differentiation the Blastula will become equivalent to 
the Planula. 'To distinguish this stage from the true Blastula, 
which consists only of uniform cells, it may be called the 
“ Diblastula” (See Pl. V. fig. 4). 
The same process of differentiation seems also to be very 
common among Insects; but it is in these somewhat masked 
by the presence of a nutritive vitellus. But if we stick to the 
principal characters of the case now expounded (namely, 1, the 
occurrence of a one-layered vesicle, and 2, the mode of differ- 
entiation of the germ-lamella), the first processes of insect- 
development might also be explained in an analogous fashion. 
‘These first processes are well known since the works of Zad- 
dach, Weissmann, and Mecznikoff. The differentiation of the 
germ-lamelle in Insects has been carefully investigated by 
Kowalevsky. Kowalevsky, in his investigations, however, 
has arrived at the conviction that the inferior germ-lamella 
of Insects constitutes a peculiar formation, and cannot be com- 
pared with that of the Vertebrata, He compares the subse- 
quently formed dorsal tube of Hydrophilus and the Phryganeidee 
with the intestino-glandular lamella of the Vertebrata. I can 
by no means share this anit To me the phenomena of 
the formation of the germ-lamelle in the Ascidia appear to be 
so in accordance with those of LHydrophilus, that I certainly 
can find no obstacle to regarding the inferior germ-lamella of 
Hydrophilus as homologous with that of the Ascidia (and 
therefore also of Amphioxus). We have seen that in the As- 
cidia the differentiation of the inferior germ-lamella is effected 
by some cells of the Blastula (the cells of the entoderm) be- 
rinning to distinguish themselves from the others (the cells of 
the exoderm). According to Kowalevsky’s researches this 
differentiation in Hydrophilus commences ina perfectly similar 
way. In order to make it easier to see our way with regard 
to the homology of these two formations, I have given two 
figures from Kowalevsky’s memoirs (Pl. V. figs. 4 and 5). 
~ In the Ascidia a vesicle consisting of uniform cells is first 
of all produced from the segmentation-cells; in the Insects 
also the same vesicle occurs, differing from the former only 
by its being filled with vitellus. In the Ascidia the differen- 
tiation of the germ-lamellz is brought about by some cells of 
this vesicle beginning to distinguish themselves from the 
others, and thus forming the foundation of the inferior germ- 
