404 Dr. O. Maas on the Interpretation 



larger cells, together with spicules, which forms the surface 

 only at the hinder pole, and in addition makes up the interior 

 of the larva. In fixation, which takes place with the anterior 

 pole, the small flagellated cells come to lie in a reversed 

 position in the interior, and the whole remaining mass grows 

 round them. From the former arise the flagellated chambers 

 and the efferent canals in part, while from the cells of the 

 latter are formed all the remaining constituent parts of the 

 sponge. 



The separation of some of the elements has already been 

 completed in the larva, so that two kinds of cells can be 

 recognized in it and spicules are formed in quantities ; but 

 other elements first become difl'erentiated after the meta- 

 morphosis. The two kinds of cells in the large-celled mass 

 are, first, such cells as are provided with a nucleus and 

 nucleolus and contain deposits of unequal size, and, secondly, 

 cells the nucleus of which shows a network and which con- 

 tain a uniform protoplasm. From the former arise the 

 amoeboid wandering cells, which, as is known, give rise to 

 the genital products ; while the latter, with uniform proto- 

 plasm, have various destinations. After metamorphosis they 

 for the first time separate into the cells of the outer covering 

 and into the contractile elements, which come to lie in the 

 parenchyma of the intermediate mass ; they are thus identical 

 with the " ectoderm," the " cellules contractiles " of Topsent. 

 The separation takes place relatively late; even during the 

 metamorphosis the " mesodermal " muscle-cells and the 

 " ectodermal " covering-cells cannot be distinguished from one 

 another, especially at the marginal parts ; their separation 

 first becomes distinct with the formation of the canal-system. 

 In these siliceous sponges also the contractile elements often 

 form whole tracts ; the diftcrentiation, however, never goes 

 so far as in the horny sponges; the "ectoderm," i. e. 

 covering-cells, never lose their contractility, and throughout 

 life look histologically very similar to the corresponding 

 elements in the intermediate mass. 



On this account there is no ground for designating this 

 covering and the contractile elements as ectoderm simply, as 

 Topsent has done, even after I have shown their common 

 derivation. The spicule-forming cells and the amoeboid 

 wandering cells might just as well be termed ectoderm. 

 It is true that they are separated in the embryos much earlier 

 than the muscular elements, but this is a difference of degree 

 and not of kind. 



On the whole the circumstances are instructive under 



