624 MURIEL ROBERTSON AND K. A. MINCHIN. 



contents and sometimes a minute central granule (fig. 30, cell 

 on the extreme right). This body is sometimes nearer the 

 blepharoplast, sometimes nearer the nucleus, Vjut usually it 

 lies at a level midway between the neck and the main body 

 of the cell or in the neck itself; its significance is doubtful. 



In addition to the vacuoles, the cytoplasm almost always 

 contains one or more coarse refringent granules of irregular, 

 angular form and yellowish-brown colour. They are lodged 

 in any part of the cell and are often present in the vicinity of 

 the blepharoplast. They prol)ably represent excretion-grains. 

 After the irou-hagmatoxylin stain they become darker, but 

 still retain their characteristic yellowish-brown tint, and can 

 be easily distinguished from chromatin grains. No other 

 enclosures, as a rule, are to be found in the collar-cells, but 

 occasionally they contain large rounded bodies (figs. 31-35 

 and 50, 51), which stain deeply with iron-hajmatoxylin and 

 nppear to be of the nature of organisms, though whether they 

 represent parasites or food ingested by the cells is difficult to 

 say. In some parts of the sponge they are found more com- 

 monly than in others, and in one case (fig. 34) no nucleus 

 could be made out in the cell ; it may, however, have been 

 cut off in the section. 



(2) Preparations for Division. — Before the nucleus 

 begins to show any of the changes in its minute structure 

 which initiate mitosis certain events take place in the cell, 

 namely, the migration of the nucleus bodily from the base to 

 the summit of the cell, the disappearance of the flagellum, 

 and the division of the blepharoplast. As a general rule 

 these three events take place in the order named, but not 

 invariably, so that a number of different combinations ai-ise 

 in different cases. 



The migration of the nucleus is always the first sign that a 

 collar-cell is about to divide, and this peculiarity is a great 

 aid to the study of the division, since in a section of the 

 sponge which shows the collar-cells cut vertically those that 

 are dividing or preparing to divide arrest the attention at 

 once, even with a comparatively low power of the microscope. 



