266 bulletin: museum of compakative zoology. 



Fig. 1) has increased very much in size and is altered in appearance. 

 The deutoplasm completely fills the region which in the young ovocyte 

 was occupied by the granular cytoplasm. It is impossible to note the 

 structure of the cytoplasm at this stage because it is so scanty and so 

 crowded in between the yolk spheres. The nucleus is large, usually 

 roundish, and centrally located. It is limited by a definite membrane, 

 which may be irregular in outline in poorly fixed material. 



The nucleolus is a thin-walled vesicle filled with a non-staining fluid. 

 The chromatin has entirely disappeared, or is located in the form of a 

 crescent on one side of the nucleolus. This increase of the non-staining 

 portion of the nucleolus is brought about by the gradual confluence of 

 the vacuoles, which at first usually arise on one side of the nucleolus 

 (Plate 2, Figs. 7, 8) ; the deeply staining chromatin diminishes in pro- 

 portion to the non-staining material. In a number of cases observed, a 

 clear region within the nucleolus enclosed a deeply staining mass, which 

 was the remnant of the chromatin of the nucleolus. Coe ('99, Fig, 13) 

 shows a similar condition in the ovum of Cerebratulus. 



The nucleolus is filled with a network of fibres that are loosely inter- 

 laced. These have on them deeply staining masses, which are especially 

 noticeable where two fibres cross. This condition is prominent in eggs that 

 have been stained with iron-ha3matoxylin. There may or may not be 

 present one or two plasmosomes. When a strong plasma stain is used, 

 the appearance of the nucleus is entirely changed (Plate 2, Fig. 8). 

 The fibres are distinguished with difficulty, and the region seems to be 

 occupied by fine granules uniformly distributed. Heidenhain ('93, 

 '94) terms these two substances, which are thus differentiated by a dis- 

 tinctively nuclear and a distinctively plasma stain, basichromatin and 

 oxychromatin, respectively. This distinction is very evident in Haminea, 

 the full-grown nucleus presenting different appearances according to the 

 method of staining employed. 



In these changes I have been unable to detect any evidence of a cen- 

 trosome. Nor is there any distinguishable polarity to the egg as it lies 

 in the follicles. The nucleus is in the centre, and does not take up an 

 eccentric position during these changes, nor even at the beginning of 

 maturation. 



2. Matuhation Changes. 



(a) Before Deposition. 



Centrosome. Notwithstanding the fact that a large number of persons 

 have studied the mitotic phenomena in Mollusca, all, with the exception 



