262 CONK LIN. [VOL. II. 



represent the egg and sperm nuclei which have not yet lost their 

 individuality. This cannot be demonstrated in Crepidula, for 

 the reason that this double character is not apparent at every 

 stage in the nuclear cycle, but it is extremely probable, as the 

 following observations will show : 



1. In the first cleavage the germ nuclei do not fuse but 

 remain distinct throughout the prophase, and even in the meta- 

 phase they are represented by separate groups of chromosomes 

 (Figs. 1,2); in the early anaphase these groups of chromosomes 

 can no longer be distinguished, though I think they must still 

 remain separate, for the nuclei are clearly double in the imme- 

 diately following late anaphase and telophase (Figs. 3, 4). 

 The position of the partition wall in these double nuclei corre- 

 sponds to the plane of contact between the germ nuclei; the 

 egg nucleus always lies more or less above the sperm nucleus, 

 and in the telophase of the first cleavage one-half of each 

 double nucleus overlaps the other half to a greater or less 

 extent (Figs. 1-4). It is probable that the upper half repre- 

 sents the egg nucleus, and the lower half the sperm nucleus, 

 and in all the later cleavages it is probable that the half of the 

 nucleus which lies nearest the animal pole is from the egg, and 

 the other half from the sperm. 



2. The groove which is found on one side of the nucleus in 

 the telophase of the first cleavage (Fig. 4) persists well into the 

 resting stage, and a corresponding groove is found in the same 

 position in the prophase of the second cleavage (Figs. 5, 6). 

 The central spindle for the second cleavage lies in this groove 

 (Fig. 6), and thus the amphiaster actually lies in the only plane 

 in which it would be possible to halve the two parts of the 

 double nuclei. This very fact shows that each half of a double 

 nucleus is represented in the daughter-nuclei, and it strongly 

 suggests that the two parts of the daughter-nuclei are derived 

 directly from the corresponding parts of the mother-nucleus 

 (cf. Figs. 6, 8). The fact that the central spindle lies in the 

 groove separating the halves of the nucleus has been observed 

 in the first, second, third, and fourth cleavages, and is undoubt- 

 edly a general phenomenon. There is no reasonable ground 

 for doubting that the two parts of every double nucleus are 



