MUSEUM OF COMPARATIVE ZOOLOGY. 531 



physico-chemical alteration of the protoplasm emanating from the cen- 

 tral area, is probably incontrovertible ; at least there is a physical 

 alteration of the protoplasm, and it first becomes apparent at the centre 

 of the aster ; but this is rather a description than an explanation of the 

 appearances. 



I have already dwelt upon some features of the astral phenomena 

 which seem to strengthen the position maintained by Flemming, — that 

 the asters repi;esent a structural condition of the protoplasm ; but that 

 simply implies a greater stability in the nature of the rays — a closer 

 approximation to a solid condition — than is generally maintained, and 

 offers not the least explanation of the cause. 



The substance which composes the central portion of the asters ac- 

 companying division was uniformly described by the earlier observers as 

 an accumulation of homogeneous protoplasm, and as such it always ap- 

 pears in the living Qgg. Flemming was the first to show that a portion 

 of this " central area " is differentiated as a corpuscle capable of a slight 

 degree of staining, and for that reason he took the corpuscle to be the 

 beginning of a new nucleus. 0. Hertwig has also recognized in all his 

 studies the existence of a stainable corpuscle occupying the centre of the 

 area, and has reproduced it in nearly all his more recent figures with 

 almost diagrammatic uniformity as a very minute body in which the 

 spindle fibres terminate. Strasburger has represented nearly the same 

 condition in his revised studies of segmentation in animals. In the 

 opinion of both these observers, the centre of the aster is occupied by a 

 visible portion of the old nucleus. The evidence that this corpuscle is 

 nuclear substance they find in the constancy with which it is stained, as 

 well as the fact that it forms the tip of the old nucleus when the latter 

 is drawn out into the form of a spindle. 



My own studies lead me to believe that different reagents are not uni- 

 form in their effects, and I would refer to this some of the various condi- 

 tions in which the corpuscle has been exhibited by the preparations in 

 the case of Limax. It is quite improbable that all the variations are 

 thus referable, but it cannot be doubted, I think, that certain acids are 

 much more likely than others to make visible a differentiated central 

 corpuscle. In acetic-acid preparations the whole area has generally ap- 

 peared nearly homogeneous and usually not well defined, but sometimes 

 (Figs. 22, 25) quite sharply limited and free from every trace of internal 

 structure ; in a few instances, as though composed of a flocculent mass 

 (Fig. 55). In certain stages of the archiamphiaster (Figs. 43, 48, 50) 

 it has been occupied by a large more or less flattened corpuscle, which 



