506 E. A. MINCHIN. 



cases, at least, the cells approach one another after immigra- 

 tion to form the trios. The next stage in the development is 

 the division of each cell of the trio into two cells, giving rise 

 to a figure which may be termed the sextet (PL 38, fig. 4, 

 coriacea; PI. 39, fig. 17, Clathrina, sp. dub.; and fig. 22, 

 embryo of falcata). The division of each actinoblast takes 

 place in such a way that the plane of cleavage is parallel to 

 the inner and outer surfaces of the body-wall, so that the two 

 daughter-cells are placed one above the other when seen in a 

 surface view of the body-wall. Hence the two cells arising 

 from the division of a single actinoblast may be termed the 

 inner and outer formative cells respectively. The whole sextet 

 is made up now of two superposed cell-trefoils, if the phrase 

 may be allowed ; so that when examined in surface view, three 

 cells with their nuclei can be made out at the higher focus, 

 and three others, exactly similar to the first and imme- 

 diately below them, at a lower focus. 



In spite of much searching I have not been able to make 

 out the manner in which the division of the nuclei takes place. 

 Indeed, though I have studied very carefully the actively 

 growing portions of the sponge, cell division in the tissue cells 

 generally remains a mystery to me. I have never seen any 

 karyokinesis, nor have I come across any stages of direct 

 nuclear division. It is not uncommon, however, to find stages 

 intermediate between the trio and sextet, — that is to say, two 

 nuclei in a cell as yet undivided. In PI. 39, fig. 22, is figured 

 a stage from an embryo of Ascandra falcata in which one 

 cell of the trio has completely divided, while in the other two 

 the division has not gone beyond the nucleus. It, is, more- 

 over, impossible in surface views of a sextet to say for 

 certain whether the division is complete between the upper 

 and lower cells ; the sextet shown in PI. 39, fig. IT, is an 

 instance of this. 



The sextet is now ready to secrete the spicule, and the first 

 preparation for this event appears to consist in a fusion of the 

 formative cells at the centre of the sextet (fig. 4) ; if not 

 actually fused, the cells are at least in such close coutact that 



