Dr. W. Salcnsky on flackers Gastrsea Theory. 3 



room to doubt tliat 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 coui'se this can- 

 not be proved by facts. 



If the theory be coiTCct 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 Gastruhi-stage should actually occur very 

 frequently in the ontogenetic development of animals ; or if it 

 is not of such general occurrence (for examj)le, 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 Gastnila-stage does not occur as 

 such must be deduced and elucidated from this ; for the im- 

 portance of the Gastrcea theory is by no means proved only by 

 our detecting the Gastnda-sta.ge in some representatives of the 

 different t}^es of animals. What is required of the Gastrea 

 theory must therefore consist (1) in the actual proof of the 

 occurrence of the Gastrula-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 Hackel, 

 deraonsti'ate the occurrence of the Ga^fruIa-stdigQ in different 

 animal types. 



I. Factual demonstration of the Gastraea theory. 



The Gastrcea is defined by Hackel in the following words : — 

 " The Gastrcea is a spherical or elliptical body, with a stoma- 

 chal cavity and a mouth-opening, the stomach-wall of which 

 is formed by two (lijferent 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-stSLge 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 a priori from the fact that the Coelenterata 



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