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The essential structural feature that differentiates the two types of 

 collar-cells is as follows. In all Leucosoleniidae, and also in all 

 Sycons, that I have had an opportunity of examining, or that have been 

 accurately described, the flagelluni arises from a basal granule which is 

 in direct contact with the nuclear membrane. The nucleus itself is more 

 or less pear-shaped and the flagellum appears to arise as a direct con- 

 tinuation of its pointed end ; as a consequence of this arrangement, the 

 nucleus is at or near the upper end of the collar- cell, generally quite 

 close to the origin of the flagellum (fig. 3). If, however, the basal portion 

 of the Hagellum, that is to say, the portion within the cell, is drawn out, 

 the nucleus may come to lie deeper down in the cell, as shown in 

 Dr. Hammer's tigures (Taf. XXVI, figg. 64 — 671, in which the con- 

 nection of the flagellum and the nucleus is clearly depicted. In some 

 cases also the collar-cells of this type are very shallow, scarcely exceed- 

 ing in vertical height the length of the pear-shaped nucleus itself; in 

 such cases the nucleus is necessarily at or near the base of the cell, 

 without losing its characteristic relation to the flagellum. 



On the other hand, in all C'lathrinidae that I have studied, I 

 have found that the flagellum arises from a basal granule situated at 

 the surface of the cell, in the centre of the area enclosed by the base 

 of the collar, and in no connection with the spherical nucleus, which 

 occupies a position at the base of the collar-cell, as far removed as pos- 

 sible from the base of the flagellum (fig. 1). This arrangement occurs 

 with, so to speak, military regularity, and is very striking in sections of 

 a partially contracted sponge, in which the collar-cells are beginning 

 to be pressed one against the other, and are consequently more colum- 

 nar in form (fig. 2). There is only one exception to this rule, and that 

 is a very remarkable one, to which I have already drawn attention in 

 former works; namely, that when a collar-cell of a Clathrinid is about 

 to multiply by division, the nucleus first travels up to the apex of the cell, 

 immediately below the base of the flagellum, and there undergoes divi- 

 sion. I have not worked out the division of the nucleus in detail, and 

 therefore cannot state whether the dividing nucleus conies into relation 

 with the flagelluni: but I am inclined to think that the basal granule 

 or blepharoplast of the flagellum plays the part of a centrosome in the 

 division of the nucleus. However that may be, the apical position of 

 the dividing nuclei is easily seen, and, by its sharp contrast with the 

 resting nuclei, renders it an easy matter for the eye to pick out the 

 dividing collar-cells in a section. The young collar-cells resulting from 

 a recent division also have their nuclei at first in an apical, Leiicosoienia- 

 like position, but as they increase in size the nucleus assumes its cha- 

 racteristic basal position. 



