INTRODUCTION 



ft 



sees that in this mode of formation, which cannot be com- 

 pared to the plan of gastrulation by invagination, the gastrula 

 cavity arises from the cleavage cavity. 



Apparently a transition between the formation of the entoderm by 

 delamination and by polar ingression is effected by a kind of entoderm 

 formation which has been observed by Metschnikoff in different Hydroids, 

 and which is designated as multipolar ingression {i.e., ingression from all 

 sides), in which single cells of the blastoderm migrate into the blastoccele 

 from different points of the surface and here form an entodermic cell- 

 mass. Nevertheless the process of forming entoderm by delamination 

 remains, in contrast with the other types of entoderm formation, some- 

 what isolated and unexplained. 



Closely related to delamination is a kind of entoderm formation which 

 was formerly held to be of frequent occurrence, but whose range of dis- 

 tribution has become more and more restricted by careful investigation of 

 the individual cases. They are the cases in which the blastomeres present 

 no radial arrangement about a point within and no definite relation to a 

 cleavage cavity. Such a stage, which is an apparently irregular solid mass 

 of cells, without cleavage cavity, has been designated as morula ; and it 

 is assumed that by a rapid division of the cells at the surface an outer 

 cell-layer is differentiated from the inner cell-mass, so that here also the 

 separation of ectoderm from entoderm would be brought about by a 

 splitting off which takes place uniformly over the entire circumference. 

 We shall see that examples of such a mode of origin of the two primary 

 germ-layers are still ascribed to many Hydroids and Anthozoa, though 

 probably the greater part of the cases referred to this method can be 

 reduced to epibolic gastrulation, in which event the morula- stage, as being 

 a Schema founded on erroneous assumptions, would have to be omitted. 



Even though the last-mentioned modes of entoderm formation, re- 

 stricted as they are to a few kinds of Metazoa, place many difficulties in 

 the way of the conception that there is uniformity in this process, it is 

 probable that more careful investigation may succeed in bringing them 

 into accord with the less aberrant types already mentioned. 



We liave seen that the chief axis of the gastrula- stage 

 unites the anterior or apical (animal) and the posterior or 

 prostomial poles with each other. In the lowest types of 

 the Metazoa — the Porifera, Cnidaria and Ctenophora — this 

 primitive axis becomes the permanent chief axis of the body ; 

 therefore these groups have been contrasted by Hatschek ^ 

 as Frotaxonia with the rest of the Metazoa, which he terms 



* Compare Hatschek's Lehrbuch der Zoologie. Jena, 1888, p. 40, as 

 well as p. 69 et seq. 



