296 bullp:tin: museum of comparative zoology. 



be shown to divide in one or the other of the four typical methods which he 

 figures in his text (pp. 102, 103). In Crepidula (Conklin, :01), however, 

 there is positive evidence against this view, and the changes in Hamiuea 

 do not fall under any of his four typical methods. In Figures 41, 44 

 (Plate 7) the wall limiting the centroplasm may be seen ; it gradually 

 brealvs up into small granules, which merge in the cytoplasm. The 

 new centrosomes are minute, solid, undifferentiated bodies ; they are not 

 at once surrounded by a layer of centroplasm that is limited by a con- 

 tinuous line, but there is formed about each new centrosome a new 

 cortical layer of sphere substance. The centroplasm does not appear 

 until later ; and when it does appear, it is a new formation, not a 

 product of the old centroplasm. The centroplasm of the centrosome in 

 such a stage as is represented in Figures 38 and 39 (Plate 7) finally 

 becomes indistinguishable, being diffused in the cytoplasm (Fig. 45). 

 A part of the old centroplasm may be directly transformed into the 

 cortical layer of the new sphere, but there is no staining reaction in it 

 until breaks appear in its limiting wall. While the following descrip- 

 tion by Boveri of the division of the centrosome in Ascaris may be true 

 for that species, it certainly finds no confirmation in Crepidula or 

 Haminea. Boveri (:01, p. 98) says : " Hier streckt sich das Centrosom 

 in der Richtung der Verbindungslinie der beiden Centriolen in die 

 Lange, und um jedes Centriol schniirt sich die Halfte des Centroplasmas 

 ab. Die Substanz des Muttercentrosoms schient ganz oder fast ganz in 

 die beiden Tochtercentrosomen aufzugehen, die sich alsbald zu Kugeln 

 abrunden und nun wieder von neuem heranwachsen." 



Mark ('81) was one of the first to call attention to the importance of 

 ascertaining the origin of the second achromatic figure, but not until 

 1894 was there much definite evidence on this problem. At this time 

 Heidenhain ('92) stated that the centrosome gave rise to the whole 

 central spindle. Since then have appeared observations by the follow- 

 ing persons who sustain this conclusion : MacFarland ('97) in Diaulula, 

 Van Name ('99) in Planarians, Van der Stricht ('98) in Thysanozoon, 

 Lillie ('.Ol) in Unio, and Conklin (:0l) in Crepidula. In Haminea 

 (Plate 7, Figs. 38-4.5), the formation of the achromatic figure in its 

 early stages agrees substantially with the conditions in the cases cited; 

 but Haminea differs from all of them in the breaking down of the central 

 spindle and the formation of a new one. The formation of a spindle, 

 which afterward disappears, has been observed by Griffin ('99) in 

 Thalassema, by Coe ('99) in Cerebratulus, and by others ; but in 

 all of these cases the spindle disappears before it has become as large 



