546 BULLETIN OF THE 



of the substance of the germinative vesicle with that of the nucleus of 

 the first segmentation sphere, is as complete as could be expected of 

 eggs which do not allow the demonstration of the spindle nucleus in the 

 living condition. I have been led to suspect that certain phases of the 

 metamorphosis — between the first and second maturation spindles — 

 have not yet been discovered ; but I have no evidence that such a hypo- 

 thetical stage renders this continuity any less certain. On the contrary, 

 if such an intermediate stage as I have suggested really exists, it can 

 have no other effect than to remove the nucleus of the first cleavage 

 sphere one generation further from the germinative vesicle, but does 

 not even warrant the supposition that the former contains a smaller pro- 

 portion of the nuclear substance of the vesicle than it would have con- 

 tained had the course of events been such as recent observers, who admit 

 the continuity in question, have claimed. But it is not a question of 

 the amount of nuclear substance which thus finds its way into the nuclei 

 of successive generations. The continuity is definitely and adequately 

 established by the fact that generation after generation the nuclei 

 have for their beginnings portions of the substance composing the 

 nuclei of the stage preceding. There is not the least doubt that such 

 beginnings exist in the lateral zones of fibre thickenings. The stage 

 in the process which is least satisfactorily understood is that of the 

 origin of the equatorial zone. There can be little question that it is 

 formed in the same manner in all the earlier embryonic stages at least, 

 and it is equally evident that it must in some w^ay be formed at the 

 expense of the disappearing nucleus. My own observations (Figs. 85- 

 89) support this view in a decided manner. Whether, however, any 

 portions of the old nucleus — and, if so, which — go over bodily and 

 unaltered into this forming disk, seems much less certain, flertwig's 

 studies assuredly off'er the best evidence we have that such is the case ; 

 but Hertwig has failed to demonstrate that the beaded elements of the 

 nucleolus are directly incorporated in the equatorial thickenings. He h;is 

 traced them as far as the central area of a forming aster, but that is not 

 the equator of an amphiaster. His observations only strengthen his 

 position that it is the nucleolus (and therefore "nuclear substance") 

 which is immediately concerned in the building up of the nuclear spindle, 

 and do not nejcessarily prove that it persists in the shape of fibre thick- 

 enings. I have once seen appearances (Fig. 85) which suggest that the 

 substance of these thickenings may be aggregated into definite visible 

 corpuscles in the region of the nascent spindle, independently of the 

 fibres to which they ultimately belong ; but I place little confidence in 



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