508 SUMMARY OF CURRENT RESEARCHES RELATING TO 



is never possible where both the pairing-cells (or nuclei) have had a 

 share in active cell-life or growth. 



Progamic divisions and reducing divisions, though sometimes co- 

 incident (as in Metazoa), are not necessarily associated, but may be 

 widely divided in the life-cycle where there is " antithetic alternation of 

 generations." 



The germ formed by parthenogeny or merogony can double the 

 number of its chromosomes. 



As to the role of the sperm, there has been much debate. One 

 school sees in it merely the bearing-in of the ferment that starts the 

 oosperm into development, or of a new centrosome to the oosphere 

 which has lost its own during the formation of the polar bodies. 

 Another school refuses to consider anything but the male nucleus, 

 which indeed constitutes the bulk of the sperm at its entrance. Others, 

 again, insist that however small be the cytoplasm of the sperm in 

 quantity, it is by no means negligible in quality. This third view is a 

 decided under-statement of the case. " The impulse for division given 

 by the sperm is no essential phenomenon of syngamous union ; it is 

 conditioned by the differentiation of binary sex, and is the indirect 

 consequence of that reduction of the sperm which makes its growth 

 within the egg the necessary prelude of complete fusion therewith." 



Maturation Divisions in Vertebrates.* — A. and K. E. Schreiner 

 have studied the details of chromatin division and reduction in the 

 spermatogenesis of Myxine glutinosa and Spinax niger. The prepara- 

 tion of the chromatin for division and the divisions themselves are 

 similar in both spermatogenesis and oogenesis. In the spermatogonium 

 there is the same number of maternal and paternal chromosomes, of 

 which every pair (male and female) are homologous. At the first phase 

 the chromatin of the single chromosomes divides into fine threads, 

 which by the controlling of the centriole come to lie parallel to each 

 other, slightly converging towards the sphere. The homologous chromo- 

 somes mutually attract, and the chromatin glides in long tracts till all 

 homologous chromosomes have found each other and have lain parallel. 

 They then blend more or less closely. The double threads contract, and 

 next there is a lengthwise splitting of both components. Next the 

 thread segments crosswise, so that each piece consists of two homologous 

 chromosomes, whose number on this account amounts to half the 

 number characteristic for the spermatogonium and the somatic cells. 

 By means of the first maturation division the components of the 

 double chromosomes are separated from one another. This is a reduc- 

 tion division. The second is in no way distinguished from the usual 

 equatorial division. The preparation for maturation divisions in Verte- 

 brates cannot be brought into agreement with Boveri's scheme \ for the 

 illustration of the mechanics of the reduction divisions, since here 

 reduction is by means of the first division, and the single chromosomes 

 behave in mitoses quite similarly to the daughter elements of the 

 chromosomes in ordinary mitoses, i.e. they are connected with the one- 



* Anat. Anzeig., xxiv. (1904) pp. 561-78. 



f Ergebn. iiber die Konstitution der cliromatischen Substanz des Zellkerns, 

 Jena, 1904. 



