KARYOKINESIS. 59 



sphere remains much smaller and persists for a considerable period, while the outer 

 sphere undergoes a metamorphosis similar to that which Lillie describes. The inner 

 sphere is very faint and difficult to detect in Crepidula, and it maj' be that Lillie 

 has overlooked it, or it may disintegrate sooner in Um'o than in Crepidula (cf. m\' 

 fig. 36 and Lillie's ('99) fig. 14). 



Vejdovsky ('88) found in the egg oi Rhycheliiiis immediately preceding the first 

 cleavage, a hyaline sphere, the "Periplast." within which in the course of cell-division 

 a new element, the " Tochterperiplast" ap^aears; the latter divides into two spheres 

 which represent the poles of the new spindle. During the first and second cleavages 

 a new element, the " Enkelperiplast" arises within the "Tochterperiplast." while the 

 " Mutterperiplast" degenerates or fuses with the cytoplasm. This was tlie first obser- 

 vation tending to show that the new centers arise endogenouslv within the old. 



More recently Vejdovsky and Mrazek ('98) have confirmed and extended this 

 account. They find at the poles of the first cleavage spindle, a large sphere at the 

 center of which is a deeply staining granule, the " centrosome',' (central corpuscle); 

 this is surrounded by a hyaline sphere the "' Tochterperiplast" (medullary zone), at 

 the periphery of which the rays are attached ; surrounding this is the " Mutterperi- 

 plast" (cortical zone). For the sake of uniformit}^ we shall use the terms in paren- 

 thesis in place of ^^ejdovsky's terminology. As mitosis advances the medullary zone 

 (Tochterperiplast) grows rapidly, becomes reticular or alveolar in structure and is 

 bounded by a dense peripheral zone ; the central corpuscle decreases in size and stains 

 less densely, while radiations appear around it within the medullary zone. Around 

 the central corpuscle a new medullary zone appears within the old one. The central 

 spindle is formed, after which the new medullary zone divides. Vejdovsky and 

 Mrazek consider that centrosome and periplast are persistent organs of the egg and 

 that they represent a single whole. The entire periplast, however, does not persist, 

 but the inner zones give rise to the outer ones which gradually disintegrate into the 

 cytoplasm. 



The resemblances between these observations and those which I have described 

 in the preceding pages are very striking. The only important difference between 

 my own observations and those of Vejdovsky and Mrazek is the following : A new 

 medullary zone does not form around the central corpuscle before the latter divides, 

 but onl\- afterwards, i. e., in the mollusks which I have studied the two daughter 

 centrosomes are present befoi'e a new medullar}- zone is formed. 



R. Hertwig has observed in Actinosphcerium a large " spong}" centrosome'' 

 within which a "' reduced centrosome" (central corpuscle) appears and divides into 

 two; the latter then enlarge to form new "spongy centrosomes," while the former 

 " spong}- centrosome " does not divide, but disappears in the cytoplasm. The resem- 

 blances in this case to those discussed above are too obvious to need comment. 



In Limax, Byrnes ('99) has found centrosomes which in many respects resemble 

 those of Unio, Crepidula, Aeolis and other mollusks. In the metaphase of the 

 first maturation there is at each pole a group of central granules, w' ithin a clear 

 area, which is surrounded by a broad, densely staining zone. In the anaphase the 



