37S WILSON. [Vol. IX. 



"The (once) ciliated cells surrounding the irregular spaces 

 arrange themselves so as to form a nearly continuous wall, in 

 which, here and there, an intermediary cell is found. This is, 

 however, not the permanent epithelium, for intermediary cells 

 lying outside it gradually take the place of the ciliated cells, 

 which in their turn come to lie outside the definitive epithelial 

 wall. Such ciliated cells, which have temporarily been occu- 

 pied in lining the canals, now follow the example of their 

 brethren and unite to form flagellated chambers." 



Observations such as are embodied in my PI. XVIII, Fig. 47, 

 and PI. XVH, Figs. 39 and 42, seem to me to contradict the 

 above account. The canals shown in these figures are evi- 

 dently just forming, and yet their walls are made up of 

 elements which, to judge from Delage's figures, I must con- 

 clude he would regard as amoeboid and intermediary, certainly 

 not as immigrated ciliated cells. 



Finally, the distinction which Delage makes between inter- 

 mediary and amoeboid cells, is to my own mind an artificial 

 one. His amoeboid cells evidently correspond to my forma- 

 tive cells, but I find no special place for his " intermediary " 

 group, because the plump formative cells are being constantly 

 changed into elements which Delage would class as interme- 

 diary. An instance of this is found in the development of the 

 dermal membrane, where formative cells are gradually trans- 

 formed into the slender elongated cells forming the mesoderm 

 of this membrane {ante, p. 307). 



Note. — While this paper is passing through the press, a new contribu- 

 tion to the subject by Otto Maas appears.' The author has studied a large 

 number of marine cornacuspongiae and has worked over the development 

 of Spongilla. His account of the metamorphosis for all these forms differs 

 but little from his previous account of the metamorphosis of the Esperia 

 larva. In some points Maas differs from Delage's recent conclusions. 

 Thus Maas does not find that the ciliated cells of the lan'a are engulfed by 

 the amoeboids and subsequently liberated. No such peculiar association 

 of the two kinds of cells occurs. With this I thoroughly agree, although 

 differing entirely with Maas in the general view of the metamorphosis. 

 Again Maas states that in those larvae with a "bare" posterior pole, like 



' Die Embryonal-Entwicklung und Metamorphose der Comacuspongien. Zoolog. 

 Jahrbilcher, Abth. fur Anat. und Ontogenie. Bd. VII., 2. H. 



