54 KARYOKINESIS. 



cases on record in which the centrosomes and central spindle of the first cleavage 

 form a unit structure. Even in many of those cases in which there is a division 

 of the sperm centrosome and a well-marked central spindle between the halves 

 {e.g., Pkysa, Plenrophylidia, Unio, Cerebratulns, Thalassema, Arenicola), this 

 central spindle completely disappears and the definitive spindle is formed de novo 

 between independent centrosomes. In view of the unit structure of centrosomes 

 and central spindles in other divisions, this is certainly a striking phenomenon, and 

 indicates that the centrosomes of the first cleavage are in their first appearance 

 more independent of each other than in any subsequent cleavage. It also suggests 

 a possible way of unifying the conflicting accounts as to the origin of the cleavage 

 centrosomes. 



(b). Relation of Centrosome to Cell Body and Sphere. — In the mollusks which 

 I have studied, the centrosome is at all stages in its cycle sharply delimited from 

 the surrounding cell-body and sphere. The outer zone of the mother centrosome 

 does not disintegrate and lose its outlines until after the daughter centrosomes and 

 spindle have appeared Avithin it, so that in all stages of the cell cycle there is a 

 clearly marked centrosome. In the cleavage the outlines of the centrosome are 

 most difficult to distinguish in the anaphase (figs. 59 and 67), but even at this stage 

 there can be no doubt of its sharp separation from the surrounding sphere. Only 

 in the egg and sperm asters during the approach of the germ nuclei is this separa- 

 tion completely lost. No clearly marked sperm centrosome can be recognized at 

 any stage, and the egg centrosome which is very evident during the anaphase of 

 the second maturation, and which during this period undergoes a typical transfor- 

 mation into a hollow sphere (figs. 32-36), loses its outlines and completely disappears 

 in the surrounding sphere before the union of the germ nuclei. This again marks 

 a peculiarity in the centrosomes during fertilization not found in any other cell cycle. 



The centrosomes and spheres grow simultaneously reaching their greatest size 

 in the telophase or resting period when the astral radiations are smallest; the astral 

 radiations again become prominent when the new centrosomes have moved out of 

 the old centrosomes and sphere and are growing rapidly in size. The growth of the 

 centrosomes and spheres is not coincident with that of the nuclei ; on the contrary 

 they are smallest when the nuclei are largest, viz., in the early prophase, and they 

 have nearly reached their largest dimensions when the nuclei are smallest, i. e., in 

 the late anaphase before the formation of the chromosomal vesicles {cf. figs. 3 and 

 16, 27 and 34, 53 and 59, 63 and 67). This, as well as other morphological phe- 

 nomena involved in the escape of achromatin at the poles of the spindle and the 

 coincident growth of the spheres and centrosomes, together with the changes in the 

 staining reactions of the latter, indicates that the spheres and centrosomes grow in 

 part at the expence of substance escaped from the nucleus. That this is not a 

 complete statement of the facts, however, is shown by all cases of unequal cleavage, 

 in which the centrosomes and spheres at the two poles of the spindle are always 

 equal until the constriction of the cell body begins (figs. 15, 33, 72, etc.), but 

 immediately after this they become unequal in size, and in the end are proportional 



