KARYOKINESIS. 53 



But whatever the theoretical bearings of this elimination may be, there can be 

 no doubt of the fact that in Crepidula, and perhaps in a large number of animals, 

 there is a very extensive exchange of material between the nucleus and cytoplasm, 

 and, further, that a large part of that most characteristic nuclear substance, the 

 chromatin, passes into the cytoplasm in the form of oxychromatin during every cell 

 cycle, while a relatively small portion is preserved for the purpose of reproducing 

 the daughter nuclei. 



There is thus in karyokinesis a rhythmical growth and collapse of the nucleus 

 as a whole, the new nuclei arising endogenously, z". ^., from chromosomes, within 

 the old. In fact, one might speak of these changes in the nucleus as a systole and 

 diastole (Ryder, '94), by means of which an exchange of nuclear and cytoplasmic 

 material is broucrht about. 



The nuclear membrane appears to permit the passage of materials inward but 

 not outward during the resting period, whereas the escape of nuclear material into 

 the cell is brought about by the disappearance of the nuclear membrane during 

 karyokinesis. Such a process is not universal, for in cells where karyokinesis 

 occurs very rarely, or not at all, the interchange between cytoplasm and nucleus 

 has been observed to take place, but the phenomena desci'ibed are characteristic of 

 mitosis in general. « 



2. Centrosomes and Central Spindles. — a. Structure and Metamorphoses. — 

 It is evident from the history of the centrosomes of Crepidula that throughout the 

 maturation and cleavages up to at least the 60-cell stage, the centrosomes are abso- 

 lutely continuous from one cell generation to the next, with the possible exception 

 of the fertilization stages. Of this fact there can be no particle of doubt. With 

 the exception of the earliest origin of the centrosomes of the first maturation, and 

 with some reserve as to the origin of the centrosomes of the first cleavage, I be- 

 lieve that I have seen every step in the origin and metamorphoses of the centro- 

 somes up to the 12-cell stage ; while in all the cleavages up to the 60-cell stage, or 

 even later, I have observed and drawn the centrosomes at almost every stage in the 

 cell cycle. Fortunately, this is not a very difficult thing to do, since the centro- 

 somes are so large and distinct that even during the height of the resting period 

 they can be seen in entire eggs, and their elongation to form the central spindles 

 plainly observed (see Plates V, VI). 



The principal features of the entire centrosomal cycle from one cell division to 

 the next may be summarized in a single sentence : The tninute granides at the 

 poles of the central spindle enlarge until they become hollozv spheres within whicli 

 new centrosomes and ceiztral spindles appear. The individual variations character- 

 istic of the maturation and the different cleavage stages have been mentioned in 

 detail, and need not be reviewed here ; it is sufficient to say that the history of everv 

 centrosome conforms to the above statement. 



From this it is evident that the centrosomes and central spindles form a unit 

 structure, as Heidenhain ('94), MacFarland ('96) and Boveri ('01) maintain. Only 

 in the fertilization is this not the case, and there are few, if any, well authenticated 



