No. 2] COMPARATIVE CVTOLOGICAL STUDIES. 457 



in the size of the mitoses hardly afford a satisfactory criterion 

 for deciding this point (Figs. 255-261). All the typical stages 

 of the prophase and metaphase are to be found, though only in 

 the arrangement of the chromatin, for I have been unable to 

 find either centrosomes or achromatic spindle. After careful 

 study of a large number of these dividing nuclei I find the 

 nucleolus to persist in the nucleus throughout the mitosis. 

 Further, it appears to retain its original size throughout this 

 process, without any diminution in volume. Thus the nucleolus 

 seems to be retained without change in the spirem and aster 

 stages of the prophasis. In the dyaster stage (Fig. 258) each 

 pole of the nucleus usually contains a nucleolus, so that the 

 nucleus contains two nucleoli ; and when the nuclear divi- 

 sion is completed, i.e., when in one and the same cell two 

 nuclei occur in close contact with each other, in the aster as 

 well as in the spirem of the metaphasis, each daughter-nucleus 

 has its own nucleolus (Fig. 257). Now the ovogonium contains 

 only one nucleolus, so that we must assume either (i) that a 

 division of the nucleolus has taken place during the mitosis, or 

 (2) that to one of the daughter-nuclei is allotted the whole 

 original nucleolus, while in the other nucleus a new one is pro- 

 duced. I have not seen any dividing nucleoli in these mitoses, 

 their small size being a great obstacle to their study. But I 

 should judge that such a division occurs, for these reasons : 

 (i) the nucleus of one or of both the daughter-nuclei has 

 sometimes a somewhat elongate form (Fig. 257); and (2) in 

 later stages of the ovum proper I have found dividing nucleoli, 

 and these cases would show that if such divisions take place in 

 stages subsequent to the mitosis they might also occur during 

 the mitosis. The two cases of division of the nucleolus found 

 are here figured (Figs. 264 and 265), and in each of the elongate 

 nuclei is a dumbbell-shaped nucleolus lying in the longitudinal 

 axis of the nucleus ; in these figures the two halves of each 

 nucleolus appear unequal in dimensions, but this is so because 

 neither of these nucleoli happened to lie wholly in the plane of 

 the section. I have found numerous other cases of elongated 

 nuclei, each with an elongate nucleolus without any median 

 constriction (Fig. 270). These facts would show that a division 



