Dr. W. Salensky on Hackel’s Gastrea Theory. 25 
If we regard the differentiation of the germ-lamelle as the 
chief phenomenon, and the formation of the intestinal cavity 
as a secondary one, it is clear that the Gastrula-form with the 
stomachal cavity, in these cases also, as in development from 
the Blastula, cannot be regarded as the fundamental form. 
From this brief revision of the first embryonal phenomena 
in animals it follows that the Gastrula-stage may originate 
from the Planula or Blastula in consequence of secondary, 
subsequently occurring alterations of the latter; in most 
instances it is not produced. After these observations I need 
hardly ask whether a form proper to only a few animals can 
represent the stock-form of all Metazoa, it being understood 
that in other animals we see the development take place quite 
independently of this form? ‘The reason of the incorrect- 
ness of the Gastrwa theory consists in the fact that in the 
stock-form of the Gastrewa a secondary embryonal phenomenon 
(the formation of the stomachal cavity) is placed in juxtaposi- 
tion with the primary and most important of these phenomena 
(the formation of the germ-lamellz). The incorrectness lies in 
the assumption that the Gastrula is that early state of deve- 
lopment ‘in which the embryonal animal body represents the 
simplest conceivable form of the person” (‘ Gastreatheorie,’ 
p- 17). Why are we to accept as the simplest being an 
animal which is already provided with a stomachal cavity, 
when we are acquainted with Metazoa (the accelous Turbel- 
laria) which possess no stomachal cavity? Such accelous 
Metazoa are represented by the Gastrula before it acquires the 
stomachal cavity and while it swims about as a Planula-form 
(in the Coelenterata). Hackel has placed this Planula among 
the animals which have no germ-lamelle, among the Protozoa. 
(See the synoptical table in the ‘Gastreatheorie’). Such a 
combination is quite unintelligible to me; for Hackel himself 
says, in his Monograph of the Calcispongie, that the differen- 
tiation of the germ-lamellz occurs even in this stage. It 
proves the artificiality of the idea of the “ Gastrula,” that it 
should stand as a “dividing boundary” between the Pro- 
tozoa and Metazoa. When the Planula of the Coelenterata 
acquires a stomachal cavity and a mouth, it becomes converted 
into a Ceelenterate (Metazoon); why should it as a Planula 
represent a Protozoon, if it possesses the two germ-lamelle 
which the Protozoa have not, and which are only produced 
by the process of segmentation, which the eggs or germs of 
the Infusoria do not pass through ? 
In the short revision of the first embryonal processes in 
animals we have seen that in most instances the two germ- 
