180 SUMMARY. 



Actinozoa ; and an epibolic invagination is characteristic of the 

 Ctenophora. 



If confidence is to be placed in the recorded observations on 

 which this summary is founded, and there is no reason why in a 

 general way it should not be so placed, the conclusion is inevit- 

 able that of the above modes of development the one must be 

 primitive and the other a derivative from it, for, if this conclusion 

 be not accepted, the absolutely inadmissible hypothesis of a 

 double origin for the Ccelenterata would have to be adopted. 



Two questions arise from these considerations : 



(1) Which is the primitive, delamination or invagination ? 



(2) How is the one of these to be derived from the other ? 



There is a great deal to be said in favour of both delamin- 

 ation and invagination ; but it will be convenient to defer all 

 discussion of the question to the general chapter on the forma- 

 tion of the layers throughout the animal kingdom. 



The hypoblast cells are often filled with yolk material, and 

 secondary modifications are thus produced in the development. 

 The most important examples of such modifications are found in 

 the Siphonophora and Ctenophora. 



In the simplest forms amongst the Hydrozoa there is no trace 

 of a third layer or mesoblast. The epiblast is typically formed, 

 as was first shewn by Kleinenberg, of an epithelial layer and a 

 subepithelial interstitial layer of cells. The cells of the former 

 are frequently produced into muscular or nervous tails, and those 

 of the latter give rise to the thread- cells and generative organs 

 and in some cases to muscles 1 . In many cases, amongst all the 

 Ccelenterate groups, and constantly amongst the Ctenophora the 

 epiblast is simplified and reduced to a single layer. The hypo- 

 blast undergoes in most cases no such differentiation but simply 

 forms a glandular layer lining the gastric chamber and its pro- 

 longations into the tentacles ; but in the Actinozoa it appears to 

 give rise to muscles, and strong evidence has been brought for- 

 ward to shew that in some groups it gives rise to the generative 

 organs. 



Between the epiblast and hypoblast a structureless lamella 

 appears always to be interposed. 



1 The questions relating to the generative organs of the Ccelenterata are dealt with 

 in the second part of this work. 



