PRESENT PROBLEMS IX EVOLUTION AND HEREDITY. 



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iiiquo, and stimulated by Wcisuiaiiii's prediction that spermatozoa 

 would also be Ibuud to extrude i)olar bodies, this author exanuued all 

 stages in the peculiarly favorable germ cells of the thread-worm of the 

 horse {Asc<(ris mcgaJovcphdla). 



lie made the suri)rising discovery that o^a and spermatcj/.oa are 

 formed in a substantially similar manner by rcjxatcd divisions, the 

 single difference being that the last jiroducts of division among the 

 sperm cells are eftective speinmtozoa, capable of development in fer- 

 tilization, while the last i)rodiicts of division in the ovary are, first 



Thk Matcratiox of Ova, or Foumatio.v of Polar Bodies in Ascaris. (Knmi Weismnhn afto 

 Hortzwiir.) A. original fjcnn-ccll in embryonic !r''rm-Uiy<M' — 4 clironialin nul.s: 1!, Ovnni motliercoll — 

 Srods; v'-I), First ))olar body extruded: K. S]>li;tin'4 oC tirst jiolar body. 0\iiini slill contains 4 rods; 

 F, Second i)olar l)o(ly extruded : Ovimii mat urc w it li 1! ro<ls. 



the true ova, and, second, the abortive ova. (polar cells), inca])able of 

 development. In both ova and spermatozoa the nucleus coidains but 

 cue-half the chromatin which w ty])ic:il nucleus contains; in the case 

 of A. wefjrdocepJiaht each of the germ cells contains but two chrom;i 

 somes while the normal body cells contain four. The manner in which 

 this maturation of the germ cells for conjugation is brought about is 

 beiuitifully shown in these diagrams, taken from Weismann's essay, 

 "Amphimixis." You observe that the nund)er of chromasomes in the 

 primary germ cells is four (Figs. 11 and 12, A). Then are fornu-d by 

 subdivision the ovum and si)erm "mother cells," in which the chro- 

 matin substance is doubled, so that we observe eight chrounisomes 

 The mother cells then divide and the chronmtin is reduced to tour 

 rods, a second division rai)i(lly Ibllows whereby the chromatin is reduced 



