212 SCIENCE PROGRESS. 



E. ligua, Bwk. (metamorphosis), Axinella Crista-galli 

 n.sp. (metamorphosis), Myxilla rosacea O. S. (from the egg), 

 Gelliusvarius, Bwk. (metamorphosis), and Chalinulafertilis, 

 Keller (from the egg), and a few observations, are detailed 

 upon the larvae of some other species and the development 

 of Euspo?igia officinalis, Hircinia variabilis, and Spongilla} 

 Delage has investigated the metamorphosis of Spongilla 

 fiuviatilis, Lbkhn., Esperella sordida, Bwk.. Reniera densa, 

 Bwk., and Aplysilla su If urea, F. E. S. 



According to Maas the segmentation is more or less 

 uniform in all the species. The first two cleavages run 

 meridionally and at right angles to one another, and 

 divide the egg into four equal segments. The third 

 furrow is equatorial and separates the ovum into four 

 smaller and four larger blastomeres, an inequality which 

 is maintained from this stage onwards. A segmentation 

 cavity, virtually present at the eight-cell stage, is more dis- 

 tinct in the later stages, especially when the inequality in 

 the blastomeres is more pronounced. In other cases it is 

 less distinct, and may become obliterated by blastomeres 

 pressing into the interior. The embryo soon becomes 

 distinctly two-layered, owing to the more rapid division 

 of the peripheral cells. At first the cells at one pole are 

 smaller than those at the other, but later the embryo be- 

 comes surrounded by the smaller cells and then consists 

 of two distinct layers : (i) a peripheral layer of small cells, 

 with clear protoplasm and small, deeply staining nuclei filled 

 with a close framework of chromatin ; (2) an inner mass of 

 larger cells, filled with coarse granules (yolk) and containing 

 each a vesicular nucleus with nucleolus. In Chahnula 

 and Myxilla the layer of smaller cells is only one or two 

 cells deep and does not completely envelop the inner mass, 

 but leaves the latter exposed at one pole. In Hircinia, 

 however, the inner mass is completely covered, the layer 



1 In his former paper on the development of Spongilla (Zeitschr. f. 

 wiss. Zool., bd. 1., 1890) Maas gave a very different account to that 

 which he now believes to be correct. He then described the ectoderm of 

 the adult sponge as arising from the ciliated cells of the larva. 



