400 Dr. O. Maas on the Interpretation 



(jerminal layers or from two only. F. E. Scluilze has 

 frequently given prominence to this distinction between the 

 layers of the adult and of the embryo, and has said tliat 

 sponges can only be regarded as triploblastic animals if, in 

 the undifferentiated embryo and before histological sepa- 

 ration, three cell-layers, distinct from one another but undiffe- 

 rentiated in themselves, could be distinguished — a state of 

 things which, to my knowledge, has not yet been demon- 

 strated with certainty in any sponge. However that may 

 be, in the adult sponge at all events, since the lead given by 

 Schulze, the covering layers of epithelium have to be sharply 

 distinguished from the third layer, the enclosed connective- 

 tissue substance with its various contained elements. 



In this triple division, which is founded on the relative 

 position of the layers and is supported by histological disco- 

 veries, a correction has recently been made by Topsenf^, 

 which at first sight appears not unimportant, but which, it 

 seems to me, makes little fundamental alteration. This 

 author distinguishes, to begin by summarizing his chief 

 results in this respect, four kinds of cells — referring first 

 to the boring sponges and then to the Halichondrise — 

 namely (1) cellules contractiles, (2) vibratiles, (8) conjonc- 

 tives, and (4) digestives pigmentees. The first and second 

 constitute, according to him, the ectoderm and endoderra, the 

 others the mesoderm. By contractile cells he understands 

 those which, as he points out, from their position and 

 appearance have hitherto been regarded as ectoderm and 

 endoderm or as " fibres," the contractile fibre-cells of the 

 mesoderm. When he further says that the latter are 

 the only elements which have been made answerable for 

 the contractility, he does not at the same time take into 

 consideration the fact that contractility and mutability of 

 form have also been usually accorded to the epithelial 

 ectoderm cells. For how can authors otherwise have formed 

 a conception of the opening and closing of pores ? Moreover 

 there is to be found in the literature a whole series of special 

 examples, in which reference is made to alterations in the form 

 of the ectoderm cells — by Lieberklihn in Spongilla, by F. E. 

 Schulze in Sycandra, by myself in the young and by Weltner 

 in the adult SiJongilla^ by Vosmaer in Myxilla, and so on. 

 Further, it is not justifiable to comprise offhand in one group 

 the cells of the external skin and the contractile elements in 

 the interior, however similar the two kinds of cells may appear 



* Topsent, E., " CoDtribution a I'Jitude des Clionides," Arcli. Zool. 

 exp(5r. V. bis Suppl. 



