268 A. RICHARDS. 



though this derivation is perhaps thinkable. The account here 

 given aims rather merely to present the conditions as they are 

 found in the fish eggs which I have studied, and to point out that 

 these facts may have significance for current cytological and 

 genetic speculation. 



Conklin in his studies on Crepidula (1902) has perhaps fol- 

 lowed out the history of the chromosomal vesicles, or karyomeres, 

 as he calls them, more completely than any other observer. Of 

 the behavior of the Crepidula chromosomes he says: "In large 

 cells where the divisions succeed one another at short intervals 

 the chromosomes begin the growth characteristic of the daughter 

 nuclei, the absorption of substances from the cell body, before 

 they have fused together, whereas in small cells or cells which 

 divide only at long intervals the chromosomes fuse before the 

 absorption of achromatic material begins." 



'The history of the nuclear changes during the cycle of divi- 

 sion may be summarized as follows: (i) The chromosomes, 

 consisting of chromatin enclosed in a linin sheath, divide and 

 move to the poles of the spindle where they partly surround the 

 spheres. (2) Here they become vesicular, the interior of the 

 vesicle becoming achromatic, though frequently containing a 

 nucleolus like body, while the walls remain chromatic. (3) The 

 vesicles continue to enlarge and then unite into the 'resting 

 nucleus'; the nuclear membrane is composed of the outermost 

 walls of the vesicles, while the inner walls stretch through the 

 nucleus as chromatic partitions; the chromosomal vesicles from 

 the egg and sperm nuclei remain distinct longer than those from 

 the same nucleus. (4) The chromatin of the inner alveolar 

 walls then aggregates into threads, giving rise to a 'chromatic 

 reticulum,' though the linin still preserves, for a time at least, 

 the alveolar structure. (5) The chromatin of these threads then 

 separates into spherules, which are connected together by linin 

 threads; these spherules vary in size, and at first are solid, and 

 stain alike. (6) They become hollow and are differentiated 

 into oxy- and basichromatin. (7) In the first maturation, each 

 of the basichromatin spherules,' or bodies, grows into an indi- 

 vidual chromosome; in the cleavage, the basichromatin spherules 

 unite into several linear series, thus forming a segmented spireme. 



