KARYOKINESIS. 69 



while still maintaining some of the propositions named above, acknowledges that 

 his opposition to the view that the intra-nuclear spindle of the Protozoa is the 

 homologue of the metazoan centrosome was not justified. The fact that so careful 

 and far seeing an investigator as Boveri should find cause to reverse his position on 

 this question lends additional weight to this idea. 



Boveri compares in detail the micro-nucleus of a ciliate infusorian in the spin- 

 dle stage, the ovocytic spindle of Ascaris, the nuclear spindle of Opa/ma, which 

 does not fill the whole of the nuclear cavity, the ovocytic spindle of Diaulula, 

 which lies entirely outside of the nuclear cavity and in which centrosomes have 

 differentiated at the poles of the spindle, and finally a type such as Ascaris (cleav- 

 age stages) in which the central spindle connecting the centrosomes has almost 

 entirely disappeared. In this series is shown the supposed steps by which the cen- 

 trosome is differentiated at the poles of the spindle-shaped figure (netrum) and by 

 wlncli the latter comes to lie outside of the nuclear cavity and separate from the 

 chromosomes. 



How entirel}- my observations on Crepidula and other gasteropods accord with 

 the general homology suggested by these different investigators can be seen at a 

 glance at my figures. Compare for example the cleavage centrosomes of Crepidula 

 (text fig. IV and Plate IV, figs. 69-76) with the micro-nucleus of Paramivciuyn 

 (Her twig) : ' 



(1) In tlie resting stage both are reticular spheres; (2) As the division begins 

 they become spindle shaped and the reticulum is drawn out into spindle fibres ; 

 (3) At the poles of this spindle pole bodies (plates) appear, the spindle and pole 

 bodies forming a unit structure (Heidenhain) ; (4) In this process the centrosomes 

 of Crepidula repeat some of the very stages which the authors named above assume 

 to have occurred phylogenetically, i. e., the reticular spindle when first formed shows 

 no sharply differentiated body at its poles ; later a centrosome appears at each pole, 

 whether as a new differentiation or from a granule before indistinguishable from 

 the others I cannot decide. 



In one respect, however, there is an important difference between the centro- 

 somes of these gasteropods and the micro-nuclei of Paramcecium or other Ciliata, 

 viz. : in the former the new spindle figure (netrum) is formed within the old centro- 

 some, the outer zone of which disintegrates. This I'esembles the observations of 

 Hertwig on Actinosphaerium, but is unlike the division of the micro-nueleus of the 

 Infusoria where the membrane persists throughout the division. 



On the other hand the disintegration of the centrosomal membrane in gastero- 

 pods precisely resembles the behavior of the nuclear mem)>rane among Metazoa ; 

 this membrane which is composed of substances similar to, if not identical with the 

 formed achromatic substance (oxychromatin and linin) dissolves and disappears 

 during mitosis, just as the centrosomal membrane does. Both nuclear and centro- 

 somal membranes, like their contents, undergo similar changes in staining reactions 



' There is also a striking resemblance between the form of division of the central corpuscle in the 

 first maturation of Crepidula and the division of the " Nehenkurper " in the swarm spores of Paramceba 

 (Schaudinn '96). 



