736 CALKINS. [Vol. XV. 



resting stages as a deeply staining granule in the nucleus (see 

 p. 7); that it divides and gives rise to the two granules 

 whose history I have given; that these finally come to lie in 

 the cytoplasmic sphere where they separate as the amphiaster 

 is formed, one going to each daughter-sphere, where mantle- 

 fibers connect them with the chromosomes. 



The nuclear origin of the centrosome has been observed in 

 a number of forms. Thus, in all Protozoa hitherto described, 

 with the possible exception of Ettglyplia, Noctihtca, and Para- 

 nioeba, the centrosome, or its equivalent, is intra-nuclear in 

 origin. In the Metazoa the well-known results which Brauer 

 obtained in the case of Ascaris vicgalocephala tmivalens have not 

 been duplicated for other forms, although a number of obser- 

 vers have been led to think that, as in Ascaris, the centrosome 

 is intra-nuclear. Among these may be mentioned Balbiani 

 ('93), Julin ('93), Mathews ('94), and Carnoy and Lebrun ('97). 

 O. and R. Hertwig have long maintained that the centro- 

 some is primitively intra-nuclear, having been differentiated 

 originally from part of the nuclear substance. There is a 

 very important difference, however, between the intra-nuclear 

 centrosome of Metazoa and Protozoa. Both Brauer and 

 Mathews, for example, describe the centrosome as surrounded 

 by a sphere, whereas in Noctihtca the sphere lies permanently 

 outside of the nucleus. ^ If my observations are correct, sphere 

 and centrosome in Noctiliica must, therefore, have an independ- 

 ent origin. As already indicated, however, there are so many 

 chances for error in tracing the centrosome in Noctiliica 

 through resting stages that this conclusion must remain an 

 open question until further research throws more light upon it.^ 



1 Mathews's statement of this process is as follows : " At maturation the cen- 

 trosomes are first accurately to be distinguished as two (at a very early stage 

 apparently as one) deeply staining, small, but distinct and characteristic, granules 

 lying side by side either in the nuclear membrane or immediately without it, 

 and invariably on that part of the vesicle nearest to the surface of the egg. Occa- 

 sionally one of the granules appears before the other and migrates some distance 

 from the nucleus before the second appears. In cases where they both lie clearly 

 outside of the nucleus, the nuclear membrane is invariably broken behind them " 



(P- 324)- 



2 Except for its decided staining qualities this intra-nuclear body might be con- 

 sidered a nucleolus and so be used as evidence in support of views of Karsten 



