KARYOKINESIS. 37 



and 70. These nuclei are very similar to those figured and described by Calkins 

 ('98) in Noctiluca and by R. Hertwig ('99) in Actinosphcprium, Avhere the basi- 

 chromatin is aggregated' at one pole (" Hauptpol ") and the oxychromatin at the 

 other (" Gegenpol ").^ 



In all cases the oxychromatin granules or reticulum completely disappear as 

 such, though this may not happen until after the spindle is well formed, e. g., fig. 

 55. Wilson ("95) maintains that a portion of the chromatin (oxychromatin) is 

 transformed into linin in Toxopneustes, and Griffin ('99) holds the same view as to 

 Thalussema; see also Lillie (1901, ]). 250). I have no doubt that this is the case 

 also in Crepidula, where many of the oxychromatin granules are arranged on the 

 linin fibres and are here dissolved and apparently transformed into the substance of 

 the fibres (see text figs. XVII, XVIII). The further history of the achromatic sub- 

 stances will be followed under the head of the mitotic spindle and spheres. 



The basichromatin is transformed into chromosomes in the manner already 

 indicated (p. 3(j). In no nucleus in Crepidu/a have I ever been able to find a single 

 continuous spireme thread. The chromosomes are formed by the union into a linear 

 series of the chromatin spherules or from portions of the chromatin reticulum, but 

 from the first there is a large number of these segments, though I cannot determine 

 whether the number is the same as the final number of chromosomes. Perhaps this 

 method of formation of chromosomes without a preceding sjiireme is to be looked 

 upon as a modification due to a precocious segmentation of the sjjireme. 



In the early prophase of several cleavages, particularly the first division of the 

 first quartette, the chromatin is aggregated into a dense mass at the center of the 

 nucleus, leaving a peripheral zone inside the nuclear membrane Avhich contains no 

 chromatin, text fig. XXIX. Such nuclei resemble in appearance the "synapsis" 

 stages (Moore, Montgomery) of spermatogenesis. This condition is the result of 

 the aggregation of the chromosomes, a phenomenon which occurs in every prophase, 

 while the resemblance to the synapsis is due merely to the persistence of the nuc- 

 lear membrane for an unusually long time. 



c. Separaiion of Chromosomes and Formation of Daughter Nuclei. The 

 chromosomes, which are at first widely scattered through the nuclear cavity, text 

 figs. XVII and XVIII, are first drawn into the equatorial plate and then transported 

 to the poles of the spindle in the usual manner. 



The sjjlitting of the chromosomes in the first cleavage, however, greatly resem- 

 bles a heterotypic mitosis. In tliis division many of the chromosomes are shaped 

 like rings, ellipses or triangles, and the parts of these figures lying in the equator 

 grow thinner and thinner, the chromatic substance aggregating in the portions of the 

 chromosomes turned toward the poles, until only a faint linin thread is left com- 

 pleting the otherwise open rings or triangles, fig. 56. I am not sure that this type of 

 division of the chromosomes in the first cleavage occurs in all eggs, since I have 

 found it in only a few cases and have been unable to find it in others of apparently 

 the same stage (cf. figs. 56 and 57). 



'Montgomery (1900) has rendered these names into the convenient English terms "central 

 pole" and " distal pole," which terms I shall adopt in this paper. 



