DIVISION OK COLLAR-CELLS OF CLATHRINA CORIACEA. 631 



the cell, with the result that the young flagella come to point 

 vertically upwards. After complete division the form of the 

 two daughter-collar-cells undergoes a change, becoming- 

 elongated, in the vertical direction, so that the cell as a whole 

 acquires a slender columnar form, with a shallow collar sur- 

 rounding the short flagellum at the upper end (figs. 29, 30, 

 48). A curious feature of these stages, both those in which 

 cleavage of the cell is taking place (figs. 25-28a) and those in 

 which division is recently completed (tigs. 29, 30, 48), is that 

 they are found in the sections at a higher level than the rest 

 of the epithelium, as shown in figs. 30 and 48; the bases of 

 the young cell are on a level Avith the flanges of the ordinary 

 resting collar-cells. This peculiarity is very marked when 

 the recently divided cells have assumed the columnar form ; 

 they project so much above the general level of the collared 

 epithelium that they become very conspicuous objects in the 

 sections of the sponge, and are consequently very easy to 

 find. Later they appear to push their way down amongst the 

 other epithelial cells, and so find their normal level (fig. 49). 

 The nuclei of the young collar-cells, at first compact masses, 

 soon become looser in texture ; the karyosome reappears and 

 nucleus acquires the structure of the ordinary resting nuclei, 

 from which it differs only in its smaller size. In osraic- 

 picrocarmine preparations the young nuclei show the marginal 

 clear zone very distinctly (figs. 48, 49). Immediately after 

 division the nucleus is at the apex of the collar-cell (figs. 29, 

 30, 48), but it now begins to migrate towards the base of the 

 cell (fig. 49), and so resumes the position characteristic of the 

 resting cell. The collar and flagellum grow to their full 

 length, and the latter arises from a basal granule or blepharo- 

 plast which, as is clear from the development that has been 

 described and depicted, is one of the two centrosomes of the 

 nuclear spindle in the mitosis, derived from the division of 

 the resting cell. 



