28 



to purely physical causes due to the position of the embryo which, 

 he states, invariably hes mth the granular pole in close proximity 

 to the body of the sponge, so that a wall of si)icules presses against 

 it, and so prevents the now rapidly prohferating granular cells 

 from occupjdng more space outwardly — hence the formation of 

 the pseudogastrula, at which stage he supposes the embryo to 

 be set free. This, however, hardly meets the case in G. compressa, 

 at least, for in this species a later stage is present in the tissues 

 of the parent sponge, showing a further increase in the number 

 of granular cells and their subsequent evagination to form a more 

 or less globular embryo. This being the case, the spicules are 

 evidently not able to retard the outward growth of the gTanular 

 cells. The embryo hes immediately behind the gastral epithehum, 

 which becomes bulged out by the growth and finally ruptured to 

 set it free. Thus when the space between the wall of spicules 

 and the flagellate chamber becomes too narrow to contain the 

 embryo, the proliferating granular cells pressing on the sj^icules 

 will simply push the opposite pole more closely against the gastral 

 epithehum, so causing it to bulge out more and more until the 

 hmit of its elasticity is reached, and it spHts, setting free the embrj^o 

 into the flagellate chamber. The fact that the pseudogastrula 

 stage occurs I take to be due rather to reduction in the pressure 

 of the contents of the segmentation cavity,, as it is a strikingly 

 noticeable fact in sections containing large numbers of ova and 

 embryos that, while unsegmented ova and embrj' os at, or just past, 

 the pseudogastrula stage may be very plentiful, the intervening 

 stages are comparatively rare. This x^omts to a period of very 

 rapid growth fron the beginning of segmentation up to the forma- 

 tion of the pseudogastrula — indeed so rapid that the expansion 

 of the segmentation cavity proceeds more cjuickly than the rate 

 of absorption of fluid from outside, hence a drawing in of 

 the less convex pole, i.e., the granular cells and the production 

 of the ]3seudogastrula. 



As these invaginated cells increase numerically they become 

 differentiated into two types — a single outer layer of hghtly stain- 

 ing cells with definite nuclei and distinct outhne, and an inner mass 

 of closely packed elements filled with granular matter and of ill- 

 defined outhne. These cells are evidently concerned with the 

 storing up of food reserves for the free swimming period, and as 



