518 E. A. MINCHTN. 



continuous at the oscular margin with the general fiat epi- 

 thelium covering the exterior. 



The mother-cells of the adventitious rays — the gastral 

 actinoblasts, as they may be termed — also originate in ways 

 which, though essentially similar, yet differ in details in the 

 two regions of the sponge body just mentioned. In the first 

 case a porocyte, which is actually performing the functions of 

 a pore, divides and gives off a cell which becomes a gastral 

 actinoblast. In the second case one of the granular epithelial 

 cells which line the oscular rim, and are destined to form 

 pores, gives rise to a gastral ray without ever having been 

 functional as a pore. Thus in all cases the gastral actino- 

 blast arises from a porocyte; but while the commonest method 

 is for the porocyte to become a functional pore-cell, which in 

 its turn gives off an actinoblast, we have in a particular region 

 a layer of porocytes giving origin to functional pore-cells on 

 the one hand, and to actinoblasts on the other. 



For demonstrating these facts Clathrina contortaisa 

 particularly favorable object, on account of the yellowish- 

 brown colour which characterises its pore-cells in preparations 

 made with osmic acid and picrocarmine. The porocytes are 

 in consequence very sharply marked off from other cells, and 

 it is very easy to trace them. It is only necessary to lay out 

 a piece of the sponge wall with the gastral surface uppermost, 

 and if the piece selected contains gastral rays in an early stage, 

 there is no difficulty in finding them. By the study of CI. 

 contorta I was able to confirm and to extend observations 

 which I had made a year earlier upon Clathrina, sp. dub. 



I will commence with the more usual mode of origin. PI. 

 42, fig. 49, shows in CI. contorta a young triradiate system, 

 lying at a deeper level, below the collar-cell layer. Imme- 

 diately over the young spicule is a coarsely granular cell, the 

 gastral actinoblast, wedged in amongst the collar-cells and 

 still in continuity with a pore-cell close at hand. The pore- 

 cell and the actinoblast resemble each other exactly in the 

 characters of the nucleus and cytoplasm. The nucleus of the 

 actinoblast lies over the centre of the spicule, and as yet no 



