240 



takes place — that the reduction must be effected. A reduction is nothing 

 more than an undoing of the union effected at a previous conjugation, 

 but by this it must not be concluded that there is any intention of 

 supposing it to be a separation of all the male- parental chromosomes 

 from all the female-parental ones. The facts of heredity , as Weis- 

 mann has proved, go to show that the process is more complicated, 

 and an excellent discussion and explanation of it have been furnished 

 by Haecker in his reply to Strasburger and elsewhere. The theo- 

 retical mode of the undoing has quite recently been proved by an 

 able investigator in the case of the Copepoda ^). 



Rückert has shown that in this group the reduced number is 

 brought about by a transverse division of what seemed to be half the 

 normal number of chromosomes, and that in the ripening of the egg 

 this takes place in the formation of the second polar body. He states ^) 

 that the reduction in the number of chromosomes before fertilization 

 is attained by the united action of two processes. 1) It is initiated 

 before the maturation, perhaps at a very early period, by the 

 suppression of a transverse division of the chromatin loop, in conse- 

 quence of which the chromosomes remain attached in pairs or 

 couples. 2) It is accomplished in the second division of the ripening 

 by the passage of the chromosomes of each pair to opposite poles ^). 

 He goes on to say that the first process alone leads only to a pseudo- 

 reduction, the true number of chromosomes persisting, being only 

 masked, and therefore capable of reappearing. The process , however, 

 appears necessary in order that the subsequent reduction should be 

 effected, A theoretical explanation of this has been attempted above. 



1) J. RtJCKEET, Die Chromatinreduction bei der Reifung der Sexual- 

 zellen. Ergebnisse der Anat, und Entwickelungsgesch., Bd. 3, 1893, p. 

 517 — 583. 



2) Summary on p, 582. 



3) It is worthy of notice that Faemeb has recently stated the follow- 

 ing facts concerning the reduction in plants. Two features characterise 

 the karyokinesis of the spore-mothercell in Hepaticae. The first of these 

 is that the number of chromosomes is reduced to one half as compared 

 with antecedent mitoses in the sporophyte, and this reduced number is 

 apparently retained in the gametophyte. The second point is that the 

 spore- forming mitoses are what Flemming has termed "heterotypic" in 

 character. (J. B. Faemee, Spore-formation and Karj^okinesis in Hepaticae. 

 Ann. of Botany, Vol. IX, June, 1895, p. 363 — 364) These facts appear 

 to agree absolutely with what Eückeet found in Copepods, but of court^e 

 in the one case (animals) the reduction occurs at the "ripening" of the 

 sexual products, in the other (plants) at the spore-formation. 



