586 GRIFFIN. [Vol. XV. 



In the fourth place, by reason of the varying accounts of 

 different authors respecting the tetrads or quadruple groups, 

 the nature of the so-called maturation divisions is still involved 

 in doubt. Boveri('87) (ovocyte of Ascaris) and Brauer ('93) (sper- 

 matocyte of Ascaris), ('92) (ovocyte of Branchipiis) agree that 

 the tetrad arises by a double longitudinal splitting of the spi- 

 reme thread, thus giving no reduction in the Weissmannian 

 sense. Other investigators, however (notably vom Rath, '92 and 

 '94, in insects, and Riickert and Hacker in copepods), describe 

 the process as consisting of one longitudinal and one transverse 

 division of the primary chromatin rod, thus giving the theo- 

 retically required reduction. Here the numerical reduction 

 (pseudo-reduction of Riickert) results from a suppression (or, 

 more properly speaking, a postponement) of one-half of the 

 transverse divisions of the spireme thread. According to some 

 authors, however {e.g., Wilcox, '95; Korschelt, '95; and Calkins, 

 '95), the spireme segments into the normal number of chromo- 

 somes, and the numerical reduction is accomplished by a sub- 

 sequent conjugation or association in pairs of the chromosomes. 

 In some types, again (notably many plants, Elasmobranchs, etc.), 

 no tetrads occur, and the chromosomes persist in their original 

 rod-shaped or ring form. A number of more recent authors, 

 working upon these forms, fail to find any evidence of reduc- 

 tion, and believe the maturation divisions to involve simply two 

 successive longitudinal divisions of the chromosomes {e.g., 

 Farmer, '95, in plants ; Meves, '96, in the spermatocytes of the 

 salamander; and quite lately. Miss Sargant, '97, and Strasburger, 

 '97, in Lilium). 



The reduction question has been so often reviewed of late, 

 that I may dispense with detailed references to the work of 

 others at this point. My own observations, I believe, leave no 

 reasonable doubt that in both forms studied a true reducing 

 division occurs, and in this respect the Echiuroids {Thalassema) 

 and Lamellibranchs {Zirphaea) fall into line with the insects 

 and copepods. 



I wish to take this opportunity to express to Prof. E. B. 

 Wilson my sincere appreciation of his interest, advice, and 

 continued encouragement. 



