Dr. W. Salensky on Hiéichel’s Gastrea Theory. 3 
room to doubt that the ancestors of the latter also, at earlier 
periods of the earth’s history, passed through the Gastrula in 
the commencement of their ontogenesis.” Of course this can- 
not be proved by facts. 
If the theory be correct it must be in accordance with the 
facts and explain them. If it is of so much significance in the 
elucidation of the phylogenetic connexion of animals, we must 
expect:—1. That the Gastru/a-stage should actually occur very 
frequently in the ontogenetic development of animals ; or if it 
is not of such general occurrence (for example, if it is over- 
leaped in the ontogeny of certain animals) some of the conse- 
quent phenomena and the analogies in the development of 
different animals must at once show us that this stage really 
formerly existed and has merely been overleaped. 2. If the 
theory is of so much significance for the elucidation of the true 
interpretation of the ontogenetic import, the development of 
those animals in which the G'astrula-stage does not occur as 
such must be deduced and elucidated from this ; for the 1m- 
portance of the Gastrwa theory is by no means proved only by 
our detecting the Gastrula-stage in some representatives of the 
different types of animals. What is required of the Gastrea 
theory must therefore consist (1) in the actual proof of the 
occurrence of the G‘astrula-stage in the ontogeny of different 
animals, and (2) in the actual proof of its significance in the 
explanation of the ontogenetic phenomena. If this were the 
case, all complicated phenomena with which we are acquainted 
by observation must find a much better explanation in this 
theory than in previous conceptions. 
Let us turn first to the facts which, according to Hickel, 
demonstrate the occurrence of the Gastru/a-stage in different 
animal types. 
I. Factual demonstration of the Gastreea theory. 
The Gastreais defined by Hiickel in the following words:— 
“'The Gastrea is a spherical or elliptical body, with a stoma- 
chal cavity and a mouth-opening, the stomach-wall of which 
is formed by two different cell-layers, the inner, non-vibratile 
gastral lamella or entoderm, and the exterior, vibratile dermal 
lamella or exoderm.” This definition is so clear and distinct 
that we may at once recognize the Gastrula-stage if it exists 
in the ontogeny of an animal. 
Let us commence our revision of embryological facts with 
the ontogeny of the Coelenterata. That in these the Gastrula- 
stage is remarkably widely diffused and plays a very important 
part follows & priord from the fact that the Ccelenterata 
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