60 KARYOKINESIS. 



central granules divide into two groups and a spindle appears between them and 

 within the " centrosphere." From my own observations I am convinced that both 

 the central clear area, with its contained granules and the denser zone which sur- 

 rounds it, belong to the centrosome, which, when fully formed in the anaphase, 

 consists of central corpuscles and medullary zone, the latter bounded b}^ a narrow 

 line or layer of granules. If this be correct the spindle in Limax arises within the 

 medullary zone, as in many other cases. 



Linville (1900) has observed a similar centrosome in Limax and other Pulmo- 

 nates, around which is a cortical zone of radiating structure. He has not followed 

 the metamorphosis of the centrosomes in detail, but his figures give evidence that 

 the history of the centrosome in these Pulmonates is not different from what is 

 known in other moUusks. 



Finally, it seems quite possible to interpret most of the multitudinous forms 

 of centers which have been described in the eggs of various animals in accordance 

 with the Van Beneden-Boveri idea, as extended and defined hy Van der Stricht, 

 and particularly by Boveri (1901), provided that the remarkable changes in the 

 structure of the centrosome from prophase to telophase be kept in mind. In 

 mollusks the centrosomes are characterized (1) by the great breadth and density 

 of the peripheral portion of the centrosome which, about the middle stage of 

 mitosis, forms a dense ring or sphere surrounding a clear area, and which in all 

 stages sharply separates the centrosome from the surrounding sphere, (2) by the 

 fact that the centrosomes grow to an unusual size during mitosis, and (3) by the 

 origin of the entire amphiaster of one generation within the centrosome of a pre- 

 ceding one. Possibly the second and third of these characteristics are the results 

 of the first, since the sharp boundary of the centrosome at all stages make it 

 unusually easy to recognize the great growth of the centrosome and also the place 

 of origin of the new centrosomes and central spindles. 



Boveri's ('01) masterful contribution on the nature of the centrosome reached 

 me some time after my paper had been completed, and I have therefore been unable 

 to make the extended use of it which I could have desired. In broad outlines my 

 conclusions as to the centrosome are fundamentally like those of Bovei'i. The one 

 most important point of difference between us is that Boveri considers the centriole 

 as a differentiation of the centrosome, perhaps a continuous and persistent struc- 

 ture, around which a portion of the centroplasm always remains to form the new 

 centrosome. On the other hand, I hold with R. Hertwig ('99) that the centriole 

 gives rise by growth to a centrosome, within which a daughter centriole differenti- 

 ates, i. e., the centriole undergoes in its cycle of development a metamorphosis into 

 centrosome and daughter centriole. In each generation the outer zone of the cen- 

 trosome is thrown off, while the new centrosomes and central spindle come from 

 the center of the old. There is thus a kind of endogenous formation of centro- 

 somes, as Vejdovsky and Mrazek maintain. 



Since receiving Boveri's paper I have carefully re-examined the critical stages 

 in my preparations to see whether I could have overlooked an outer zone of centre- 



