14 MICHIGAN ACADEMY OF SCIENCE. 



in a ''body" were thus freed, this leading to true sexuality as exhibited 

 in dimorphic sex-cells that by their union give rise to a fertilized egg. 



While the struggle for existence, as just indicated, may well have been 

 the active stimulus to asexual dimoi'phism and thus to sexual differentia- 

 tion, there are oilier reasons that ajfpcar even more cogent. 



If bulk is advantageous uneijual binary division will favor the larger 

 resultant cells. Unequal division will noiinally occur through variation^ 

 or, in a cell containing meta]»1asm a smaller cell will be cut off at the 

 u])per pok\ >.'a1ui-al seh-ction will then favor the larger cell and greater 

 and greater inetjuality will result until the lesser half represents a 

 polar body only. It should be noted that this polar body is not com- 

 jtarable to the cell jir<tduced by multicellular division. In each case an 

 eipial distribution of the nucleus occurs, be it into two or many parts, 

 but the cytoplasm in the case of the egg division is nearly all retained 

 in one. 



It should be said that before this the diverse reproductive ca]>abilities 

 referred to have arisen, ]»art instead of half the body being set free in 

 reproduction, metazoa h.ave become established and the reproductive 

 differentiated from the somatic cells. 



As a result of the very unequal division described, the larger cell 

 will be left with little free energizing power for it has gained but 

 infinitesimal excess of surface and has lost a i)ortion of the metaboliz- 

 ing nucleus. It is in the best possible condition for maintaining its 

 life processes at a very low ebb until a season of abundance comes to 

 it again. By a subsequent process, however, the extrusion of the second 

 I)olar body with half its nuclear nuichinery (which see later) and the 

 tachygenetic change which followed, it has evolved to its own destruc- 

 tion unless some outside power endow it. 



On the other hand, the minute cell, the ]>roduct of multi]»le division, 

 is active, su])erabundantly vital and needs but to obtain food to grow 

 and develoj) into a new organism. 



Could, then, the passive "ovum'' of a Volvox ever have maintained 

 itself, tiding over, in its hibernation state, the rough ]»laces in its 

 experience and completing its develo])ment as favorable conditions arose 

 again? Perhaps, Init it must have been at a serious disadvantage as com- 

 pared with what, by a step further, it has gained. 



Could the smaller, motile ''spermatozoon" maintain itself by finding 

 new and congtMiial surroundings? Possibly, but it must have been 

 grievously handica]>ped by its lesser variability, at least. If, however, 

 the two, by combining their forces could secure both the desirable ends 

 of variability and abundant nutrition how satisfactory the outcome! 



It has been demonsti-ated iPoveri* ;ind Delagefl that the essential 

 act in fei-tilization is the union of the speruuitozoon with the o^■ulary 

 cyto])h»sm. The union of the germ-nuclei is a secondary phenomenon. 



What, however, has caused the spermatozoon to seek and enter the 

 ovum? It is significant that the s]>ei*m nucleus, as soon as it enters the 

 i^^ii, begins to grow, and, as we see, at present, continues to do so until 

 it reaches a size equivalent to that of the ovular nucleus itself. The 

 i^iifi: seems, as is but natural, to furnish a pabulum foi- it that is exactly 



* Sitz d. (Jesellscliaft fiir Morphologic und I'liyslologie zur Miinchen. Sitznng am 16 Juli, 

 1899. 



t Comptes Rendus, txxvii, 528-531. 



