12 Dr. W. Salensky on Héckel’s Gastreea Theory. 
In the stock of the Vertebrata a Gastrula-stage occurs only 
in Amphioxus lanceolatus. 
From this brief summary we may conclude that the diffusion 
of the Gastrula-stage in the ontogeny of animals is limited to 
the following—the Ccelenterata (with the exception of the 
Ctenophora), the Echinodermata, probably some Nemertina, 
Lumbricus, Sagitta, the Ascidia, perhaps some Mollusca (?), 
and Amphioxus lanecolatus. 
Il. The significance of the Gastrula-stage. 
Having shown, in the preceding section, that the Gastrula- 
stage is not so generally diffused in the ontogeny of animals 
as Hiickel asserts, we have already in part furnished evidence 
that its importance in ontogeny is not so great as Hiickel 
states. Nevertheless it may be very justly objected to this 
notion that, although the Gastrula is not of such general oc- 
currence, it may yet, as a stock-form, play an important part 
in the elucidation of the phylogenetic relations of animals. 
The Gastrula-stage might be overleaped in some animals, 
or obscured by some secondary ontogenetic phenomena. We 
ought then to recognize this overleaping of the Gastrula- 
stage from some other embryonal phenomenon. The Naw- 
plius-stage, which may with perfect justice be regarded as 
the stock-form of the Crustacea, may be seen in the most 
different orders of that class; in the most diverse representa- 
tives of these orders we may, with the greatest certainty, 
derive from this stage the further changes, the progressions 
and retrogressions of development. Such are the require- 
ments that we must lay upon the Gastrula-stage if we are 
to regard the Gastrea as the stock-form of the Metazoa. 
We ought therefore to recognize its occurrence in many 
animals, and be able to read in the development of the animals 
the history of gradual changes from this stock-form. This, 
however, we cannot in reality do. We know no single case 
in which, the Gastrula-stage being wanting, the later em- 
bryonal phenomena can be elucidated by it; we do not even 
know of any instance in which the primitive intestine is re- 
placed by a later one. On the contrary we always see that, in 
those cases in which the Gastrula-stage occurs, this primitive 
intestine becomes transformed into the permanent intestine, 
and the primitive mouth remains in these forms (except in 
Sagitta) as the permanent mouth. Why are we to charac- 
terize this intestinal cavity as the primitive intestine, when in 
no instance can we see a secondary intestine? But in those 
cases in which we cannot detect any Gastrula-stage (e. g. in 
the Arthropoda, Mollusca, most Vermes, &c.) we witness the 
