66 KARYOKINESIS. 



Others have observed that centrosomes disappear within the growing daughter 

 nuclei in certain cases. For example, Mead says that the centrosome left in the egg 

 at the. close of the second maturation of ChcEtopterjis is last seen " in the midst of 

 the fusing (chi'omosomal) vesicles, its position being indicated b}' the point of con- 

 vergence of the rays of its waning aster." Exactly the same thing is true of Cei'e- 

 brahdus (Coe), Thalassema (Griffin), Asterias (Wilson and Mathews) and probably 

 of other animals. In all these cases the centrosome is probably talcen into the 

 egg nucleus. 



All of these facts seem to me to indicate that the centrosome is intimately 

 I'elated to the "formed achromatic" substance of the nucleus and that, in some 

 manner, artificially produced centrosomes are formed out of this material as R. 

 Hertwig maintains. 



Loeb has found, by a series of remarkable experiments, that artificial partheno- 

 genesis may be caused in the eggs of echinoderms and of Chcetopterus by the action 

 of a variety of substances upon the eggs, and he concludes that in general this par- 

 thenogenesis is the result of diosmotic action of these substances and the with- 

 drawal of water from the egg, though other factors also enter into the problem in 

 the case of Clicetopterus. In the light of the many observations and experiments 

 which go to show that asters and centrosomes are produced from escaped nuclear 

 material, the thought suggests itself that artificial parthenogenesis maj^ be caused 

 by any method which will bring certain nuclear constituents into the cell body and 

 yet not seriously injure either nucleus or cytoplasm. 



One of the most interesting chapters of Boveri's recent work on the centrosome 

 is that in which he undertakes to account for the production of centrosomes by arti- 

 ficial means. Boveii recognizes, as have many others, the intimate relation between 

 the achromatic material of the nucleus and the centrosome. In cases where real 

 centrosomes can be produced from unfertilized eggs he holds that the}' are formed 

 by a kind of regeneration [reparation, Driesch '97) from the achromatic substance 

 of the nucleus. Not all nuclei, however, have this power, and, accordingly, Boveri 

 distinguishes between (1) nuclei which are purely nuclear in character, and (2) 

 centro-nuclei which contain a cytocenter. An example of the former is found in 

 Ascaris, and of the latter in many Protozoa, and in some Metazoa, particularly in 

 the echinoderms and in the ovocytes of many animals — perhaps of all. 



The questioi\ at once arises: What reason is there for supposing that among 

 Metazoa nuclei are divided into these two classes? Boveri himself has asked this 

 question, and he concludes that in Ascaris at least the nucleus cannot be a centro- 

 nucleus, since in certain pathological eggs, in which the spermatozoon remained at 

 the periphery or did not enter at all, the egg went through the maturation divisions 

 and the egg nucleus came to the period of the solution of the nuclear membrane 

 without a trace of fibre differentiation, of centrosomes, or of spindles or spheres. 

 He therefore concludes that the nucleus of Ascaris is a pure nucleus which has lost 

 the capacity of forming centrosomes. 



The evidence upon which such an imjDortant generalization is based seems to 



