212 THE EVOLUTION OF THE METAZOA 



The oral opening is developed later by. means of an invagi- 

 nation. The contents of such a stereogastrula is naturally not 

 an entoderm but rather an entomesoderm which was called 

 an archihiston by Steinbock— inasmuch as it still shows the 

 primitive character of a plasmodium (syncytium). It becomes 

 clear that the supposedly primitive formation of the entoderm, 

 which has been considered as typical and which takes place by 

 way of an invagination of the vegetative half of the coeloblas- 

 tula, had been evolved so to speak in the "technique of embryo- 

 genesis" from a combination of an immersion ("passing into 

 the interior") of certain macromeres, e.g., 4A and 4jB, and 

 of an epiboly (surrounding growth by means of micromeres; 

 cf. conditions in the early ontogeny of the Poljchoerus caudattts^ 

 according to Gardiner, 1895, and of Convoluta roscoffensis, 

 according to Georgevich, 1899). 



It can be given as a direct proof of the origin of Cni- 

 daria from Turbellaria that it is in the Anthozoa only 

 that we find instances of an inclination to spiral cleavage 

 which is typical of Turbellaria and which was passed over 

 from Turbellaria to Mollusca as well as to Annelida. The 

 simplification of the in -determined character of the early 

 development of Cnidaria has been mainly due to the retrogres- 

 sion of the middle body layer which was caused by their 

 sessile way of life. In this connection the role of the blastomere 

 4 ^ as that of a supplier of the entomesoderm (coeloblast, 

 according to Daw^ydoff) has been diminished so that what has 

 remained has been the irregular formation of the so-called 

 entomesoderm, the mesenchyme. 



It is characteristic, if viewed from the standpoint of a 

 general study of ontogeny (embryology), that no typical for- 

 mation of the so-called germ layers can be observed either in 

 Turbellaria or in their descendants Cnidaria. Kiihn (1914) was 

 obliged to state in connection with Hydrozoa that "... die 

 Keimblatterbildung (takes place) durch DiflFerenzierung aus 

 einem soliden, indifferenten Blastodermhaufen oder einem 

 Syncytium." Is it better not to speak here of any germ layers 



