444 R. M. PACE. 



tion of the yolk nucleus appears to vary somewhat in its 

 details in different eggs (PL 22, figs. 18-21). The fragments 

 of the yolk nucleus retreat towards the periphery of the egg, 

 and there for a time form a disconnected ring of darkly-stain- 

 ing substance, each segment of which is still surrounded by 

 a clear zone (PI. 22, figs. 22, 23). Soon after this the fragments 

 of the yolk nucleus lose their identity, and at about the same 

 time the first indications of the true yolk make their appear- 

 ance as small granules, which are scattered about in the cyto- 

 plasm (PI. 23, fig. 24, Y.). At the same time vacuolisation of 

 the cytoplasm occurs j the yolk spherules come to lie within 

 these vacuoles and rapidly increase in size until they appear 

 to occupy the entire egg (PI. 23, fig. 25). 



The whole process of the growth and disintegration of the 

 yolk nucleus and the formation of the yolk takes place some 

 time before the egg is released from the ovary. 



That the yolk nucleus is a true cell organ, and not merely 

 an appearance due to the coagulation of proteid by the fixing 

 reagent used, is proved by the fact that it is visible in the 

 living egg as a dark mass surrounding or overlying the 

 nucleus. 



The nature and origin of the clear zone or space which has 

 been described as surrounding the yolk nucleus are somewhat 

 doubtful. It may possibly contain a fluid, but all attempts to 

 prove by means of staining reagents that this is so have 

 failed ; and it would seem perhaps more probable that this 

 apparent space is only an artificial one caused by the con- 

 traction of the substance of the yolk nucleus during fixation. 

 This latter view is supported by the fact that the clear zone 

 has not been detected in the living egg, nor in those eggs 

 (PI. 23, figs. 26, 27) which have been fixed by chromo-nitro- 

 osmic acid, and also by the fact that the shape of the sup- 

 posed space corresponds so exactly with that of the yolk 

 nucleus which it surrounds. 



With the view to determine Avhether the formation of oil 

 globules, which has been described and figui-ed by von Bam- 

 beke (1) as preceding yolk formation in the case of Pholcus 



