No. 3.] THALASSEMA Ai\D ZIRPHAEA. 625 



Chromatin. 



In comparing the foregoing description of the chromatin in 

 Thalasscma and Zirphaea, we observe that, while a close par- 

 allelism exists between these forms, each complements and 

 throws light upon the other. During the growth period of 

 Thalassctna, the ring segments of the spireme greatly elongate 

 and, in the full-grown egg, become so distorted that their true 

 form is recognized with difficulty. Sometimes, probably by 

 reason of a temporary loss of staining power, even their pres- 

 ence is hard to demonstrate. Here Zu-pliaca yields important 

 evidence, for the chromosomes are very conspicuous as thick 

 rings in various stages of condensation. During prophase 

 the chromosomes in Thalassema again show clearly as open 

 rings or double rods, while in Zirpliaca the already concen- 

 trated tetrads become partially drawn out into rings. In both 

 forms the division takes place by drawing out of the chromo- 

 somes into narrow ellipses followed by one cross division 

 whereby V's result. At telophase these break at the angle 

 and do not split longitudinally, as Meves ('95) iinds in the Sala- 

 mander. The process shows most clearly in Zirpliaea, where 

 the facts demonstrate that the V's cannot arise by bending of 

 a single rod, as Miss Sargant describes in the case of Liliicm. 



The ring or double-rod chromosome is a very common type 

 in the maturation of animal germ-cells, and the rings in most 

 cases undergo a subsequent transformation into quadruple 

 groups or tetrads. The ring is probably the more primitive 

 type, and the four-sphered tetrad probably arises secondarily to 

 facilitate the transference of the chromatin masses during the 

 ensuing divisions. But in all forms heretofore studied, the quad- 

 ripartite form, when once assumed, is retained and the four 

 quarters distributed among the four daughter-cells. In all forms 

 where a reducing division occurs the elements of the tetrad are 

 so arranged that each sphere comprises exactly one-half of one 

 of the daughter-segments produced by longitudinal splitting of 

 the spireme segments. In Zirphaea we find a structure closely 

 simulating an ordinary tetrad, yet the spheres are in this case 

 not homologous with those of a quadruple group. This is 



