KARYOKINESIS. 65 



formation, and I see no evidence in the cases brought forward by Morgan to indi- 

 cate that the centrosomes or asters in all these cases may not be derived from 

 escaped nuclear material.' 



There is certainly a close relationship between the nuclei and the centrosomes. 

 The achromatic substance of the nucleus contributes to the growth of the cen- 

 trosome in every normal cell cycle (see p. 54), and it is probable that the daughter 

 nuclei in their growth resorb from the sphei-es a portion of this same achromatic 

 substance. The peripheral spindle fibres are formed out of this substance [vis., 

 linin and oxychromatin), and bear a strikmg resemblance to the central spindle 

 fibres at an early stage [c/. figs. 55 and 75). In the first maturation of the egg the 

 centrosomes or asters do not appear until substances have escaped from the nucleus, 

 as is shown by the breaking or indentation of the nuclear membrane [cf. Coe '99, 

 Carnoy and Lebrun '99, Gardiner '98. Griffin 1900, Mead '98, Van der Stricht '98, 

 Schockaert 1900), and, finally, the granular or reticular centrosome undergoes the 

 same changes in reaction to stains as does the oxychromatin and linin, being at one 

 time uniforml}- chromatic, and later uniformly plasmatic in reaction. In all these 

 respects the centrosome behaves like an isolated jjortion of the oxychromatin and 

 linin. 



A large number of investigators have observed the formation of centrosomes 

 and spheres from some of the nuclear constituents, particularly among the Protozoa. 

 (Brauer'93 in Ascaris, Riickert '94 in Cyclops, Ishikawa '94 and Calkins '98 in 

 Nocliluca, Balbiani "95 in Spirnchona, Schaudinn "9G in Acanthocystis, Hertwig '99 

 in Aciinosphceriuin, et al.). 



' Wilson's ('01) recent work on Toxopneudes< shows that asters and centrosomes may ai-ise in eggs 

 treated with Mg Ci,, not only far from the nucleus, but even in enucleated fragments. Wilson says (p. 

 542) "There is absolutely no evidence for, and the clearest evidence against, the view that the original 

 cytasters form at or near the nucleus, to migrate thence toward the periphery, or that they arise by the 

 multiplication of a single primary center." He holds, therefore, that centrosomes and asters may arise 

 de novo in the cytoplasm. Such a view, if generally true, would be fatal to the one which is set forth 

 in this paper, and it deserves more extended treatment than can be accorded to it in a foot-uote. In 

 brief, the critical questions as to Wilson's experiments are these: (1) Are the bodies in question real 

 centrosomes; (2) do they arise de novo in the cytoplasm? I am not disposed to question the fact that 

 these bodies are really centrosomes, but I am inclined to doubt the statement that they arise de novo, if 

 by that it is meant that they arise without genetic relation to other centrosomes or to the nuclei. The 

 fact that these "artificial" centrosomes may appear far from a nucleus, or even in enucleated frag- 

 ments, does not necessarily imply that the}' are wholly independent of them. The achromatic sub- 

 stance of the nucleus maybe widely distributed throughout the cell during mitosis, and I have observed 

 in the eggs of Crepidula, which have been placed in 29'-'^'/'' Na CI for several hours, that the achro- 

 matic portion of the nucleus may exist as one or more vesicles, with definite walls, quite distinct from 

 the chromatic portion. In some cases these achromatic vesicles are in contact with the chromatic one ; 

 in others they are widely scattered throughout the entire cell. Furthermore, many of these vesicles 

 (ipparenthj give rise to centrosomes. If the achromatin of the nucleus is genetically related to the cen- 

 trosome as I have maintained in this paper, and if achromatin, diffused throughout the cell, may, under 

 certain stimuli, be aggregated into vesicles, which then give rise to centrosomes, Wilson's observations 

 need not necessaril}' mean that centrosomes arise de novo. 



In all cases in which "artificial" asters and centrosomes have been produced, a large amount of 

 nuclear substance has been present in the cytoplasm. No one, so far as I can recall, has observed 

 asters in egg cells while the germinal vesicle is still intact; with the escape of achromatin from the 

 germinal vesicle, however, numerous asters and possibly centrosomes may appear in the egg. I have 

 tried by various means to produce asters in egg cells before maturation, but always without success as 

 long as the germinal vesicle remains intact. I believe that it may be laid down as a general principle 

 that escaped nuclear material is essential to the formation, of an aster, and that an aggregation of such 

 material is necessary to the formation of a centrosome. 



9 JOURN. A. N. S. PHILA., VOL. XII. 



