McCrady.] 182 [April 18, 



certainly exists an apparently irreconcilable difference between the 

 observations of Hertwig and my provisional theory as to the agency 

 of segmentation. The preference I had given to the view that the 

 cutting up of the yolk, or its appropriation by fragments, is due to 

 the ordinary spontaneous mobility of protoplasm, insinuating its ex- 

 tensions between the particles of yolk, and investing and removing 

 portions of it, discarding at the same time the notion of an attraction, 

 while I explained the appropriation of the segment spheres, and their 

 appression one against the other by the contraction of an unseen 

 protoplasm enveloping them, seems to find no support from the obser- 

 vations of Hertwig, which actually display such phenomena as might 

 legitimately be expected if the morula-forming agency were a polar 

 force. It is indeed impossible to avoid being struck by the general 

 resemblance between the arrangement of the lines of granules, as 

 observed by Hertwig about the poles of the elongated nucleus in the 

 changes preparatory to segmentation, and the "lines of force" along 

 which the iron-filings arrange themselves about the poles of an elon- 

 gate magnet. Moreover, the impression is not lessened by the con- 

 duct of the segmentation, for the plane of division between the two 

 segments when it appears does so precisely in the diamagnetic or 

 equatorial plane at right angles to the axis of the elongated nucleus 

 here representing the magnet ; and the surfaces of division everywhere 

 become at right angles to the lines of granules, which represent the 

 lines of force. Such phenomena might be regarded as indicating 

 that the division takes place along the plane of weakest polar action; 

 while in strict correlation with this explanation is the observation that 

 the clear protoplasm masses itself about the two poles of the nucleus. 

 The interpretation suggested is, that the granules follow the lines of 

 weakest polar action, while the protoplasm arranges itself in those 

 lines and places where the action is strongest. 



The hypothesis of a polar force, too, is entirely consistent with the 

 apparently causeless approximation and appression of the two seg- 

 ments one against the other, after their division is complete. For 

 the two magnets resulting from the division of a single bar magnet, 

 would of course attract each other and finally adhere together, if 

 free to do so. 



There seems therefore to be sufficient analogy of phaenomena to 

 make the exclusion of a selective polar force quite impossible in the 

 present state of our knowledge. 



