550 CELL DIVISION IN EGGS OF CREPIDULA. 



fuse to form the nuclear vesicle. Its outer achromatic layer was supposed to be 

 derived from the sheath of linin covering the chromosomes themselves. In the 

 light of the observations here recorded it is evident that an achromatic membrane 

 may be formed wholly independently of the chromosomes. This membrane is thinner 

 than the usual nuclear membrane and it lacks a chromatic layer. It is probably a 

 precipitation membrane. 



Evidently the growth of the nuclear vesicle in hypertonic solutions and 

 its ability to absorb achromatin is dependent to a certain extent upon the stage 

 of development of the nucleus, and more particularly of the chromatic membrane. 

 In figs. 171 and 172 the egg nucleus has remained small and its vesicle contains 

 only chromosomes and a very clear fluid, in spite of the fact that this nucleus lies 

 in the cytoplasmic area, the nuclear membrane is also thin and achromatic; the 

 sperm nucleus, on the other hand, has grown to a great size and contains most of 

 the achromatin (chromatic sap) which was previously scattered through the 

 cell. If the egg nucleus has passed a certain stage in its development from the 

 chromosomes which were left in the egg after maturation, it also grows, as the 

 sperm nucleus does, and if the resting period is prolonged both germ nuclei may 

 become very large (fig. 173). If any of the achromatin is scattered through the 

 egg as cytasters or vesicles (fig. 174) the germ nuclei are correspondingly smaller. 

 These results indicate that the substance absorbed by the nucleus from the cell 

 is a specific substance and not merely the general material of the cell body. 



In a previous paper (1912) I have shown that young nuclei grow by the 

 absorption of material from the middle zone of centrifuged eggs, and not from 

 the lighter or heavier zones. The growth and ultimate size of nuclei was found 

 to be proportional to the abundance of the material of this middle zone, and to 

 the length of time during which it was being absorbed by the nucleus. From 

 the present work it is apparent that the substance which contributes most largely 

 to the growth of the nuclear vesicle is achromatin which escaped from the nuclear 

 vesicle at some previous mitosis. This achromatin, or chromatic sap, forms the 

 interfilar substance of the spindle and asters and radiates through the cell along the 

 astral fibers; it gives rise to cytasters and achromatic vesicles; it contributes to the 

 growth of the daughter nuclei; it is probable that chromatin grows at its expense and 

 that it in turn receives certain substances from the chromatin; and finally it seems 

 likely that it is this achromatin which establishes connections between the chromatin 

 and the cell body, through which connections the influence of the chromosomes on the 

 differentiations of the cell body are exerted. 



9. Abnormal Mitosis and Amitosis. 



There are many abnormal mitoses in these experiments which are super- 

 ficiany like amitoses. Among these may be recognized the following classes: 

 (1) Those which are due to irregular separation of daughter chromosomes which 

 are scattered along the length of the spindle; in these forms a chromatic con- 

 nection is left between daughter nuclei which is at first glance strongly suggestive 



