THE EMBRYOLOGY OF THE PORIFERA. 219 



According to the account given by Maas, the process 

 by which the two layers of the larva shift their position 

 is mainly as follows. The larva fixes by the anterior 

 ciliated pole and the granular cells at the hinder pole 

 burst out, if they were not already exposed in the larva, 

 and grow round the mass of ciliated cells by a kind of 

 epibole. Especially instructive for the understanding" of 

 this process is a figure given by Maas [(2) pi. xx., fig. 

 19], representing a section of a larva of Clathria coral- 

 loides preserved a few minutes after fixation, showing the 

 inner mass Mowing out, as it were, from the hinder pole 

 and round the flagellated cells. As a result the cells of 

 the inner mass come to completely surround the flagel- 

 lated cells, which have lost their flagella and form a 

 compact mass of cells in the interior ; just as do the 

 collar cells in greatly contracted tubes of Ascetta clathrus} 

 Thus the two layers of the larva, easily distinguished by 

 the smaller and more brightly staining nuclei of the 

 former flao-ellated cells, have become exactlv reversed in 

 position. A period of apparent rest now takes place, the 

 larva having become greatly flattened, with an actively 

 amceboid margin. During this period an interpenetration 

 of the two layers is taking place rapidly. At the end 

 of this period the larva commences to increase slightly 

 in size, owing to the formation of cavities in the in- 

 terior. Lacunae arise of two kinds : the one, the future 

 exhalant canals, appear in the mass of cells with small 

 nuclei, the former flagellated cells ; the other, the future 

 inhalant canals, arise in the outer parenchyma, derived 

 from the inner mass of the larva. The flagellated cells 

 arrange themselves in little groups along the efferent 

 lacunae, each group becoming a chamber, the cells regain- 

 ing their flagella and forming collars. The epithelium 

 lining the efferent canals arises, however, from cells of 

 the inner mass of the larva, and is not, as hitherto sup- 



1 See the figures given by v. Lendenfeld (Zeitschr. f. wiss. Zool., bd. 

 liii., pi. ix., fig. 30) and myself (Quart. Journ. Micr. Set'., N.S., vol. xxxiii., 

 pi. xxix., fig. 14). 



