Hyatt.] 152 [March 5, 



has pointed out the fact, that this early differentiation of exoteric 

 and esoteric cells may be translated as due to an early inheritance 

 of a tendency to form an endoderm and ectoderm which might 

 possibly have been derived from a type in which these membranes 

 had been present. This objection can, we think, be met by two 

 considerations. The Porifera are, if we are correct, the lowest type 

 of Metazoa, and in them the succession of embryonic metamorpho- 

 ses indicates, that a true endoderm cannot be considered as repre- 

 sented by any external cells occurring in the placulate or early 

 stages of primitive types of ova. The succession in the embryo is 

 first, formation by segmentation of a placula with cells all similar, 

 then the differentiation of apical or esoteric, and basal or exoteric 

 cells. These are followed by the rounded morula, and the hollow 

 amphiblastula, and in both of these last the endoblast is formed ; 

 but, as shown by Schultze in Sycandra, this endoblast is of mixed 

 derivation, not being confined to the products of the apical or eso- 

 teric cells alone but partly formed by division of the more prim- 

 itive exoteric or basals also. There is evidently here a confusion 

 of elements which can be explained, if we are dealing with cells 

 representing a colony of Protozoa in which the exoteric cells gave 

 rise to esoteric cells above, and continued to supply the esoteric 

 layer with new recruits from their own ranks by peripheral growth. 

 This fact is, however, at variance apparently with the view that 

 the esoteric cells are in any sense descendants of true endodermic 

 cells, since as simple endoblasts they alone should be the fathers 

 of all the descendent endoblasts and endodermic cells, and not 

 stand in need of reinforcements from the exoteric cells below. The 

 esoteric cells of the amphimorula of Porifera have not descended 

 from forms in which the endoderm was fully differentiated, but 

 from forms in which this membrane was only in the first confused 

 stages of formation. The fully differentiated esoteric cells of the 

 amphimorula become invaginated in the gastrula and become the 

 rudiments of an endoderm, or a true endoblast. 



Our view is, therefore, that the esoteric cells should be con- 

 sidered as the forerunners, and not as the descendants of true 

 endodermic cells. In those cases in which an endoderm occurs in- 

 ternally by delamination, like that of Geryonea, and of Eutima, the 

 internal cells arising from the inner ends of the external cells must be 

 considered as due to the fusion of the earlier stages, and should be 



