12 Dr. \V. Salensky on IliickeVs Gustriiia Timor y. 



In the stock of the Vertobrat.i a Gastrula-sta^e occurs only 

 in Amphioxus fouceolatits. 



From this brief summary we may conclude that the diffusion 

 of the G(istrula-sti\'j;c in the ontogeny of animals is limited to 

 the following — the Ctclenterata (with the exception of the 

 Ctenophora), the Echinodermata, probably some Nemertina, 

 Lumhri'aut, Sagitta^ the Ascidia, perhaps some Mollusca (?), 

 and Amphioxus lanccolatus. 



II. The significance of the Gastrula-sfa^e. 



Having shown, in the preceding section, that the Oastrula- 

 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 llackel 

 states. Nevertheless it may be very justly objected to this 

 notion that, although the Oa^tnda 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-stii^e 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 Nau- 

 plins-stago, 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, witli 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 Gastnca 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-furm. This, 

 however, we cannot in reality do. We know no single case 

 in which, the Gastnda-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-stago, occurs, this })riniitive 

 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 Gasfrula-Htnge (e. g. in 

 the Arthrop(i(l;i, Mi-Husca, most Vermes, &c.) we witness the 



