114 ON THE ANATOMY OF THE PLY. 



contents of the cell form a well-marked mass around the nucleus separated 

 from the cell wall by a clear space (Plate X., fig. 9 a). Sometimes one or 

 more small shining nuclei appear in the clear space, and the central mass 

 becomes nebulous, and degenerates into molecular granules. Occasionally 

 it becomes segmented into two or four, previously to its degeneration (Plate 

 X., fig. 9, b, c, d). Ultimately the cell wall disappears, and the whole 

 yolk becomes homogeneous (Plate X., fig. 8d, represents the yolk during the 

 degeneration of its segmented cells). 



The ovum has by this time attained its full size. The ovi- 

 capsule becomes separated from the cellular layer or yolk sac be- 

 neath, by a clear area corresponding to the zona pellucida, and the 

 wedge-shaped cells of the external layer of the yolk, the yolk 

 sac, become converted into hexagonal chitinous cells of pave- 

 ment epithelium. (Plate X., fig 15). 



The ovicapsule forms a bladder-like projection at this period 

 at the upper extremity of the ovum, immediately above the 

 germinal disc, and a groove commencing at the germinal disc 

 is gradually formed between the yolk sac and the yolk. (Plate 

 X., fig. 10). The germinal disc and the groove or yolk canal 

 may now be observed to be surrounded by rapidly growing 

 cells ; and a group of cells appears at the upper extremity of 

 the germinal disc. This may be distinctly seen to surround a 

 minute canal the micropyle by means of which the germinal 

 disc communicates perhaps directly with the inflated portion 

 of the ovicapsule. These changes take place before impregna- 

 tion. 



The groove or yolk canal is the only part within the yoke 

 sac which remains unaltered after impregnation, and it may 

 be observed, even after the larva has escaped from the ovum, 

 as a distinct line on the interior of the yolk sac. 



The length of time required for the development of the 

 ovum up to this point is unknown ; for, although I have 



