104 NICHOLS— SPERMATOGENESIS ONISCUS ASELLUS LIXX. [ApiiU. 



longitudinally, showing it to be divided into regions called by him 

 formative, growing and ripening zones. The formative region cor- 

 responds in Oniscus to the reserve groups of spermatogonia, the 

 growing region to the apical part of the follicle and the ripening 

 zone to the basal part of the follicle. Ishikawa's conclusions con- 

 cerning reduction have not been substantiated by recent research. 



1894. Riickert. This well-known paper concerns the ovogenesis 

 of the Copepods, Cyclops strenuus, Heterocope and Diaptomus. 



In Cyclops the number of chromosomes is 22-24. The germinal 

 vesicle shows double threads of chromatin, a longitudinal split 

 having occurred at an early period. At the beginning of matura- 

 tion these contract to double rods, whose number is the reduced 

 one and which have, moreover, become transversely split. As the 

 spindle is formed the chromosomes come to lie in the equator, with 

 the longitudinal split at right angles to the axis of the spindle. 

 The first division is thus equational. In the second division the 

 chromosomes are separated along the transverse split, and this 

 division is therefore reducing. 



In Heterocope and Diaptomus open rings are formed which, 

 through condensation, become the tetrads. The plane of the first 

 division is not so easily determined for these Copepods. In the 

 opinion of Riickert the first maturation division of Diaptomus is 

 equational. 



1895. Hacker studied the ovogenesis of the Copepod, CajitJio- 

 ca7nptus. The reduced number of chromosomes is twelve. There 

 are apparently two divisions of the ovogonia. The last division is 

 followed (i) by a transverse breaking apart of a doubly split thread 

 and a shortening and thickening of the segments so that twelve 

 double rods are produced. Some of these are transversely split 

 Or (2) the last division of the ovogonia is followed by a condensa 

 tion and longitudinal division of the thread as a whole and a sub 

 sequent breaking apart of the thread into twelve double rods 

 These become transversely split and form chromosomes correspond 

 ing to the tetrads of the first mode. In either mode the changes 

 follow immediately upon the last division of the ovogonium, and no 

 true reticulum is formed in the germinal vesicle. Since the width 

 of the chromosomes is equal to their length, it is impossible to 

 settle the question as to the order in which the longitudinal and 

 transverse divisions occur. 



1895. vom Rath describes the ovogenesis of marine Copepods 



