MATERIALS FOR A MONOGRAPH OF THE ASCONS. 479 



formative cells, suspended in a structureless jelly, which 

 makes up the greater part of the body-wall. Before we can 

 discuss the relations of the connective-tissue layer and the 

 origin of its cells it is necessary to be acquainted with the 

 other elements of the dermal layer, namely, the neuro-muscular 

 layer, the pores, and the wandering cells, in at least sufficient 

 detail to enable us to distinguish them from one another, and 

 so to trace their history. 



The Dermal Epithelium (Neuro-muscular Layer; 

 '^Ectoderm" of Authors). — In the first place let me repeat 

 the statement that the dermal layer, wherever exposed, is 

 covered by flattened epithelium, at least in the normal and 

 expanded condition of the sponge. I wish to lay especial stress 

 upon this, even at the risk of offending by repetition, since 

 reference has been made to me as one of those who deny the 

 existence of the flat epithelium, although, as a matter of fact, 

 I have never done so, but, on the contrary, have repeatedly 

 affirmed its presence. Thus in 1892 (2, p. 490, foot-note), 

 when criticising Bidder's statement that the external epithe- 

 lium — or ectoderm, as we all called it then — consisted in Ascons 

 of mushroom-shaped cells, I stated such cells to be of rare 

 occurrence in freshly preserved material of Clathrina cla- 

 thrus, '' the predominant form of the ectoderm being flattened 

 epithelium." Again, in the same year (3, p. 181) I began 

 my description of the " ectoderm " of the same species with 

 the following statement: — "This layer, the contractile layer 

 of the sponge, consists in the expanded state of flattened non- 

 ciliated cells." In the face of these very definite statements I 

 am unable to understand how Lendenfeld is able to say (1894 

 [1], p. 161), ''Neuerlich haben. . . Bidder und Miuchin die 

 Behauptung aufgestellt, . . . dass die Spongien iiberhaupt kein 

 Plattenepithel besasseu." And again in the same year (1894 

 [2], p. 508) he refers to me as one who, in company with 

 Bidder, has opposed Schulze's views as to the nature of the 

 flat epithelium. It is evident that von Lendenfeld has taken 

 very little trouble to understand the views that he criticises. 

 To clear up any misconceptions which this careless reviewer 



