No. I.] CH^TOPTERUS PERGAMENTACEUS. 31 7 



with a centrosome at each end. The new spindles form 

 directly, and the ^%^, divides into four cells. Just as the new 

 spindles arrive at the equatorial-plate stage, each of the four 

 centrosomes divides, anticipating the cleavage of the egg into 

 eight cells. The new centrosomes also travel apart during the 

 reconstitution of the four nuclei (Fig. 11). In the second 

 cleavage, as in the first, the nucleoli are dropped out into the 

 cytoplasm in the equatorial plane (Fig. 11). At every phase of 

 the nucleus in the two-cell stage, the centrosomes, with radiatiofis, 

 are perfectly clear, and lie outside the nucleus in the neighbor- 

 ing cytoplasm. This is of interest « /w/t^j of Hertwig's asser- 

 tion {Die Zelle und die Gewebe, p. 46) " dass auch mit neueren 

 Methoden und optischen Hiilfsmitteln sich Centralkorperchen 

 fur gewohnlich neben dem ruhenden Kern im Protoplasma der 

 Zellen nicht nachweisen lassen." 



In the four, eight, and sixteen-cell stages, the karyokinetic 

 phenomena are the same as in the two-cell stage, as regards 

 the behavior of the centrosomes, the nucleoli, the Zwischen- 

 kdrper, the chromosomes, and the reconstitution of the nuclei. 

 One can always count about eighteen nuclear vesicles at each 

 pole of a spindle. 



With regard to the cytogeny of Chcetopterus, it is interesting 

 to note that while the ^^^ segments in the typical annelid 

 fashion up to about sixty-four cells, including the formation of 

 the "apical rosette" (Wilson: Nereis), no "cross" is formed, 

 but the cells in question continue to divide obliquely (spirally). 

 This annelid is, so far as I know, unique in this respect. 



Nereis, Amphitrite, Clymenella, and Lcpidonotus, are, I be- 

 lieve, the only annelids in which the exact origin of a part, or 

 the whole, of the prototroch has been determined. A large 

 part of the prototroch is formed, in all cases, by the very same 

 cells, which cease to divide thenceforth. In Chcetopterus, how- 

 ever, these cells continue to divide without interruption. 

 Wilson showed, several years ago, that there is no prototroch 

 in this form. 



University of Chicago, November, 1894. " 



