THE GASTR.'EA THEORY AND ITS SUCCESSORS. QI 



from segmentation ; delamination then occurs, produc- 

 ino- a diblastula. At the same time, however, or even 

 previous to the delamination, a certain amount of dis- 

 integration of the inner ends of the cells takes place, 

 the central cavity becoming more or less filled with 

 granules of food-yolk. These are later on either ab- 

 sorbed, or pass out through the mouth -opening, which 

 is formed later, and we get a structure resembling 

 closely in general appearance a typical gastrula. The 

 fact that Kowalewsky found some food -yolk in the sup- 

 posed gastrula cavity of Cerianthus seems to indicate 

 that we have in that form not an invagination, but a 

 delamination such as occurs in Metridium. 



To sum up briefly, in the lowest Metazoa there is pro- 

 duced a solid embryo, either by immigration of certain 

 of the blastula cells or by delamination, a hollow 

 diblastula being formed in a few cases by the latter 

 process, and still more rarely invagination occurs. Ac- 

 cordingly the difficulties in the way -of the Gastrsea 

 theory are very great, and the other theories fail to 

 explain the immigration phenomena. It is hardly logi- 

 cal to take phenomena occurring in comparatively few 

 cases to be the most typical, and consequently we must 

 assume that the formation of a soUd planula and a sub- 

 sequent hollowing out of the central mass is typical in 

 the Porifera and Cnidaria. 



The Parenchymella or Phagocytella theory proposed 

 by Metschnikoff ^ seems much more in accordance with 

 the facts than any of the theories hitherto discussed. 

 He starts with a spherical hollow colony of flagellate 

 Infusoria similar to Volvox, and supposes that in it " cer- 



1 E. Metschnikoff. Embryologische Studien au Medusen. Wien. 1886. 



