Dr. W. Sak'iisky un UdckeVs Gastrsea Thconj. 'Ih 



If we regard the differentiation of the germ-lamellaj as the 

 chief plienomeuon, and the formation of the intestinal cavity 

 as a secondary one, it is clear that the Gastrula-iorm 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-stSLge may originate 

 from the Plamda 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 Gastnea theory consists in the fact that in the 

 stock-form of the Gastra-a 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 fonnation of the germ-lamellge). The incorrectness lies in 

 the assumption that the Gcistnda is that early state of deve- 

 lopment " in which the embiyonal animal body represents the 

 simplest conceivable form of the person " (' GastrEeatheorie,' 

 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 Gastnda before it acquires the 

 stomachal cavity and while it swims about as a Planula-iorm. 

 (in the Coelenterata). Hackel has placed this Planula among 

 the animals which have no germ-lamellte, among the Protozoa. 

 (See the synoptical table in the ' Gastrceatheorie '). Such a 

 combination is quite unintelligible to me ; for Hackel himself 

 says, in his Monograph of the Calcispongige, that the differen- 

 tiation of the germ-lamellge occurs even in this stage. It 

 proves the artificiality of the idea of the '' Gastrula," that it 

 should stand as a " dividing boundaiy" between the Pro- 

 tozoa and Metazoa. When the Plamda of the Coelenterata 

 acquires a stomachal cavity and a mouth, it becomes converted 

 into a Coelenterate (Metazoon) ; why should it as a Plamda 

 represent a Protozoon, if it possesses the two germ-lamellse 

 which the Protozoa have not, and which are only produced 

 by the process of segmentation, which the eggs or genns 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- 



