ELISHA MITCHELL SCIENTIFIC SOCIETY. lOI 



development and the egg development of many silicious 

 sponges. x\s in the egg embryo, there are formed in the 

 gemmule embr\'o two germ layers. In the two embryos 

 the layers are alike in many details of structure. The egg 

 larvae, again, are characterized by the absence over one 

 pole of the columnar ectoderm (Isodyctia and Desmacidon, 

 Barrois (i) ; Reniera, Marshall (2) ; Chalinula, Keller (3) ; 

 Esperia, Schmidt (4), etc. ). The account according to 

 which the endoderm protrudes at this pole, is probably not 

 correct, but it is likely that the ectoderm is only greatly 

 flattened over this region. It is this characteristic more 

 than any other, which I should pick out as a point of exact 

 resemblance between the sexual and asexual larvae. Bar- 

 rois has described the egg development of two forms, in 

 one of which (Isodyctia) the non-ciliated pole of the plan- 

 ula is never covered with columnar cells. This is par- 

 alleled by the Bsperella larva. In another form (Desmaci- 

 don), the larva has at first a complete covering of columnar 

 ectoderm (and cilia), which then disappears over one pole 

 (and much later over the whole surface). This case is 

 more or less similar to the gemmule development of 

 Tedania; in this form, the embryo has at one time a com- 

 plete covering of columnar cells (unciliated however), 

 which J^aflen out over one pole, while over the rest of the 

 body they acquire cilia. In the metamorphosis, also, the 

 gemmule development resembles the egg development, in 

 that the ectoderm of the larva is flattened to form the ecto- 

 derm of the adult. As regards the formation of the 

 chambers, canals, etc. , the egg larvae differ too much among 

 themselves to permit any such comparison as I am carrying 

 out 



Such a resemblance between the sexual and asexual 

 larvae as I have indicated, can, I think, only be explained 

 on the supposition of some essential likeness between the 

 mesoderm cells, which make up the gemmule, and ova. 



