HETEROTYPICAL MITOSIS IN NEREIS LIMBATA. 



61 



The maturation process in the egg of Nereis does not begin 

 until fertilization has taken place, and the earliest stages contained 

 in my material show the nuclear membrane still unbroken, while 

 outside of it two small asters have made their first appearance. 

 Within the large nucleus fourteen chromosomes are found scat- 

 tered around, most of them, however, lying relatively near to the 

 nuclear membrane. 



The chromosomes appear in shapes, well known from other 

 worms — Allolobophora (Foot and Strobell, 1905), Tomopleris 

 (Schreiner, 1906a) and others — forming rings and crosses of dif- 

 ferent kinds ; but they also very often appear in a more irregular 

 shape. (See earl, proph. of 1st mat. div. ; p. 62.) 



A comparison of these different chromosome forms shows 



b c d e f g 



Fig. 1, a-g. Schematic illustration of the development of chromosomes from 

 the original tetrad. Explanation in the text. 



that they all are reducible to one and the same type — to a more 

 or less elongated tetrad (Fig. 1, a) in which the four originally 

 parallel elements maybe arranged in different ways (Fig. \,b-g). 



In most cases the four elements are combined in pairs, so as to 

 give the appearance of two longitudinally split ribbons, connected 

 at one (Fig. 1, b ; chrom. 1, p. 62), or at both ends ; in the latter 

 case the chromosomes form more or less typical rings (Fig. 1 , c ; 

 chrom. 4-6, p. 62). 



In other tetrads we find the four elements connected at one 

 end, but diverging from this point in different directions (Fig. 

 1, d). Such an arrangement gives rise to cross-shaped chromo- 



