188 THOMAS IL BKYCE. 



tbe alveolar or reticular ceutroplasm. I do not presume to 

 give au opinion on the much vexed question of the persistence 

 of the centrosome as a special cell orgau^ but one thing seems 

 clear, that the centroplasm is a focus of protoplasmic 

 activity, and is ultimately to be explained on physiological 

 and not on mechanical grounds. 



Changes in tbe Germinal Vesicle Preparatory to 

 Division. — When tbe germinal vesicle has reacbed its full 

 growth the nucleolus loses its staining capacity to chromatin 

 staius; the nuclear network takes an intense stain, and the 

 cytoplasm to its very outer edge is seen to have an alveolar 

 structure. In many cases, presumably in stages close to the 

 onset of maturation, the nuclear membrane is puckered. The 

 germinal vesicle tben moves towards tbe surface, and, as long 

 ago described by Hertwig (1877) for Asteracauthion, at the 

 spot nearest the surface a protoplasmic process projects into its 

 interior (fig. 4). In osmic acid preparations the nuclear mem- 

 brane is seen to be indented and folded before tbe process ; 

 this, as it projects inwards, spreads out in every direction 

 from the neck, so that at the margins of tlie process are seen 

 sections of peniusulai and islands. In sublimate material the 

 nuclear membrane is not so sbarply differentiated, and the 

 inward folding of it is not so clearly seen. I bave no doubt 

 from my sections, sucb as shown in hg. 4, that this is a true 

 invagination of the germinal vesicle by tlie cytoplasm. At 

 tbe neck of the invagination the alveolar walls of the cyto- 

 plasm are drawn inwards towards tbe centre, but in tbe pro- 

 cess itself no very distinct fibrillar structure is at first to be 

 seen. 



Hartmann represents at this stage a very distinct aster 

 between the invaginated wall of tbe vesicle and the surface 

 of the ovum. I have not seen such an aster in my sections; 

 the wall of the vesicle is always very close to the surface of 

 tbe iigg, leaving no room for such a formation, and the aster 

 seems to form within tbe process. Sometimes tbe radiations 

 from the neck of the invagination are nuich better marked 

 than in the ovum represented, and in the }jroccss itself there 



