AIATUBATION OF OVUM IN EOHINUS ESOTTLENTUS. 189 



is a distinct suf^gestion of radiation, which is, however, very 

 difficult actually to define. The appearances sug-gest that 

 the centrosome or kinetic centre lies in the neck of the 

 invagination ; but at this stage there is not, so far as the 

 study from sections can determine, any wide-spreading astral 

 formation as in Asterias. Soon it is seen that the whole mass is 

 made up of what seem to be looping fibres. Possibly the folded 

 and puckered nuclear membrane contributes to this appear- 

 ance. In fig. 5 is represented a stage in which the chromatic 

 reticulum has become finer, and at the neck of the process is 

 an irregular mass which is destined to form the future chromo- 

 somes. All this time the germinal vesicle remains close to 

 the periphery of the egg*. 



The next stage I can determine is the one represented in 

 fig. 6. The nuclear membrane has now entirely disappeared, 

 and in the irregular mass of looping fibres there are seen two 

 asters. In each is a circular finely reticular area, the centro- 

 plasra, and from the periphery pass out in every direction 

 very delicate interdigitating fibres. Between the two asters 

 the fibres are drawn out to form an irregular spindle arrauge- 

 ment. Round this area the greater part of the nuclear 

 reticulum, which does not form chromosomes, but was i^elated 

 to the vegetative stage of the germinal vesicle, is seen merg- 

 ing with the cytoplasm, but still retaining its reticular 

 character. Between the spindle and the surface the chromo- 

 somal chromatin mass is seen. 



This description corresponds with Hertwig's original 

 account, and also in the main with Hartmann's recent repre- 

 sentation of the facts, but differs from Fol's in that he describes 

 no process projecting into the vesicle. It also differs from 

 Mathews' description of what occurs in Asterias Forbesii. 

 He describes the two centrosomes probably passing out of the 

 germinal vesicle at the nearest point to the surface of tlio 

 egg by the rupture of the nuclear membrane at that point. 

 They then pass some distance from the nucleus, and are seen 

 to have round them a faint halo of '' archoplasra." This 

 latter becomes distinct, radiations are developed, the whole 



