Nucleus of the Animal and Vegetable •• CeW 200 



constantly referred to in my later memoirs ; and I shall pre- 

 sently mention an observation made last year by Dr Harless 

 while at Triest, which affords a highly interesting proof of its 

 existence. I need not add one word to shew the importance 

 of an orifice in such a situation. 



The ovum and its ovisac arise out of a cytoblast, Plate I., 

 fig. 37. I must refer to my memoirs for the details of this 

 change,* intending here to confine my remarks to the mature 

 ovum. 



The essential part of the ovum is the germinal vesicle or 

 cell (the part not coloured in the figure last referred to, and 

 c in fig. 45) ; and more particularly its nucleus, — the germinal 

 spot. The first changes connected with fecundation, that are 

 discernible in this spot in the ovum of the Rabbit, seem to be 

 of a preparatory nature ; for they begin ante cditum^ the ani- 

 mal being in the state of heat. They are such as those de- 

 scribed in pp. 205-208, and represented in figs. 7 to 15. The 

 germinal vesicle thus fills with cells in concentric order (fig. 

 45, c) ; and now the outer layer, liquefying, becomes a sort 

 of pabulum for the second layer, which then occupies its place, 

 and so on. * 



At length the ovum becomes fecundated, by the introduc- 

 tion of a substance from the seminal fluid into the hyaline 

 centre of the germinal spot, which then passes to the centre 

 of its vesicle or cell (fig. 16). t And now the 

 contents of the cell, no longer in communica- ^^^' ^^' 



tion with the exterior by a parietal nucleus, 

 undergo changes, having sole, or at least / r\ 

 especial reference to that fecundated centre. 

 The concentric layers of cells are successively 

 and rapidly dissolved, and as speedily replaced 

 by fresh broods of cells arising in the centre. It must be 

 recollected too, that the cells in the concentric layers are 

 themselves filled with other cells, undergoing like changes. 



* Phil. Trans., 1838, Plate V., figs. 1, 2, 3, 18, 19, 20, and Phil. Trans., 1841, 

 Plate XX\^, figs. 164 to 173. 



t This figure, and several which follow it, are in outline only. 



