100 Proceedings of the Royal Society of Victoria. 



for Grantia labyrmtJdca. Concerning the inner mass of 

 granular cells, Metschnikoff continues, " Diesen Zellenhaufen 

 kann man als Mesoderm cleuten, wie es solche Larven 

 bevveisen, wo in demselben sich melirere Nadeln vortinden 

 (Fig. 13). Icli will nicht behaupten, dass die von mir 

 beschriebenen Stadien durchaus normale sind ; ich glaube 

 vielmehr, dass sie uns eine sehr verfriibte Bildung des 

 Mesoderms, resp. der Nadeln reprasentiren, welchei* V^organg 

 aber mit dem normalen qualitativ ganz ahnlicb verliiuft. 

 Wenigstens babe ich audi an vollkommen regelmassio; und 

 normal ausgebildeten Stadien eine, wenn audi bei weitem 

 nicht so stark ausgebildete Mesodermanlage wahrgenommen." 



Metschnikoff, then, has certainly seen in Sycandra 

 something closely resembling what I have found in Grantia 

 lahyrintltica, and it is highly probable that the develop- 

 ment of the two forms is almost, if not quite, identical. 

 Metschnikoff 's interpretation of the appearance of the 

 internal mass of granular cells as an unusually or 

 abnormally early development of the mesoderm is probably 

 due to the fact that in older larvse he finds this mass to be 

 no longer visible. This fact, however, is easily explained 

 according to my view of the case, which is as follows : — 



The embryo already at a very early stage lies within a 

 cavity lined by a special layer of endothelial cells. As it 

 develops it increases greatly in size, and obviously receives 

 nutriment from the motiier sponge, probably thi'ougli the 

 medium of the endothelial cells.* Balfour -|- has already 

 expressed the opinion that the granular cells of the free 

 swimming embryo are nutritive in function, and this I hold 

 to be correct, though I do not suppose that they take in any 

 food while the embryo is swimming about. I maintain 

 that the granular cells absorb nutriment from the maternal 

 tissues, increase in size, proliferate rapidly, become mechani- 

 cally invaginated as before explained, and when they have 

 done absorbing nutriment become arranged in a hemispherical 

 mass of large ovoid cells, highly charged with food granules, 

 and an investing epithelial layer (Fig. 5). The embiyo is 

 now ready to lead an independent existence, and the 

 internal mass of granular cells is, I believe, a supply of food 

 which enables it to wander for a long distance before becoming 



* Compare my account of the embryos of Stelospougos flabelliformis. — 

 Quart. Jour. Micro. ScL, December 1888. 



t " Morj^hology and Systematic Position of the Spongida."— Quarf. JoMr. 

 Micro. ScL, vol. xix., 1870 ; also, " Comparative Embryology," vol. i., p. 122, 



