364 



PRESENT PROBLEMS TIST EVOLUTION AND HEREDITY. 



to two rods, or hall" the original quantity. These last clivisious take 

 place by karyokinesis, but, as Hertwig poiuts out, they differ from 

 typical karyokinesis in the fact that tlie divisions follow so rapidly 

 upou each other that the vesicular resting period of the nucleus is 

 omitted. Thus, he suggests, is prevented an overaccumulation of 

 chromatin substance prior to the fusion of the ovum and sperm. 



lijsLB 



Spermatogenesis in awakis. — (From Wcismaiiu after Hertwig.) A, ori^iinal ucriii cell— 4 cliro- 

 matiurods; 15, speini iiintlicr coll— 8 rods; C-D, tirstdaufrliter cells wiMi 4 rods each ; E-F, tornia- 

 tioii of second daughter cells, or injitnrc siiennatozoa, with L' ro<ls cacli. 



It is evident that the polar cells arc rudimentary ova, which do not 

 possess the yolk nmss, etc., essential to development, and are divider 

 off at a very late stage, sometimes after the egg has left the ovary, 

 but are in other respects analogous to the spermatozoa. The reason 

 these polar-cells have not disappeared altogctlier in either plants or 

 animals is that they originally possessed a deep physiological impor 

 tance. As the first i)olar cell subdivides and forms two, it follows that 

 from both ovum and sperm mother cells four daughter cells are formed, 

 each containing half the chromatin substance of a normal nucleus. In 

 the ovary three of these daughter cells abort and the fourth forms a 

 true ovum; in the sperm gland, however, all four daughter cells form 

 spermatozoa. 



We may thus consider the polar-cell problem as in all ])robability set- 

 tled; the whole process is probably an inheritance or survival of a 

 primitive condition in which all four ova, like the four spermatozoa, 

 were fnlly functional. 



The relation between the chromatin and lieredity. — We have just 

 seen that the last stages in the preparation of the ova and spermato- 

 zoa for conjugation result in halving tlie number of rods in the original 

 germ cells. Now, as Hertwig and Weismann point out, one point is 



