of the Sponge Organism. 403 



elements, and after separating out the eontractile cells as of 

 epithelial origin, he regards as " mesodermal organs " proper 

 only the skeleton and the genital products. " Cellules 

 digestives pigmentdes " he docs not mention among them ; 

 but these from their function — according to the one author 

 they take up food-stuff;^, according to the other they only 

 transport them further from the digesting flagellated cells — 

 must stand in closer relation to the covering of the surface 

 and of the interior, or, rather, become set free directly from it. 

 However justifiable it may be to apply the histology, 

 especially of a primitive sponge, to the interpretation of the 

 middle layer, nevertheless we may expect still better explana- 

 tion from the developmental history. In the development of 

 Sijcandra F. E. Schulze has incidentally shown that in this 

 sponge there are present at first in any case only two germinal 

 layers, which afterwards form the three layers of the adult, 

 inasmuch as from the flagellated cells of the larva arises only 

 the endodermal system, and all remaining elements spring 

 from the larger non-flagellated cells of the embryo. With 

 regard to this it may be pointed out that he (at that time 

 surely not without intention) enumerates the differentiations 

 of this layer in the same serial order in which, as it now 

 appears, they were laid down both in the ontogeny and 

 phylogeny. He says, for example, " Shall now this layer of 

 tissue thus constituted, in which the skeletal parts arise, the 

 genital cells are formed, and in places even contractile fibre- 

 cells occur, be termed mesoderm, and its outer flat epithelial 

 covering ectoderm, or not?" He arrives at a negative con- 

 clusion, because all these elements are differentiated out of one 

 embryonic cell-layer. How this indubitable process is carried 

 out in detail has still, as is well known, to be investigated. 



In a larva, the structure and metamorphosis of which 

 appear to permit a comparison with Sycandra, it was my 

 good fortune to be able to follow * tliis differentiation some- 

 wiiat more closely, and particularly to determine how the 

 various elements of the middle layer become separate at 

 different periods of the ontogeny. The larva of Esperia (as 

 also a series of other Desmacidonidaj-larvas investigated by 

 me) consists, apart from complications of detail, in the main 

 of tioo different layers — first of a layer of small and very 

 slender flagellated cells, with minute nuclei, which lie more 

 anteriorly and make up the greater part of the surface of the 

 larva ; and secondly of a much more bulky layer of much 



* O. Maas, " Die Metamorphose von Esperia Lnrenzi, uebst Beobach- 

 timgen au andereu Schwaiumlarveu," Mitt. d. Zool. Station zu Neapel, 

 X. lid., 3 (1802). 



