36 KARYOKINESIS. 



by the hypothesis of the individuality of the chromosomes, first advanced by Rabl 

 ('85) and afterward ably defended by Boveri ('87, '88, '92). 



{b). Chromatin. At the beginning of the prophase of the first and second 

 cleavages the. nucleus contains a large number of rounded chromatin granules, which 

 are connected together by a faintly staining linin network, figs. 45-55 and 62-64. 

 These granules are at first solid bodies, but later become hollow spherules,-^ figs. 45-52, 

 and in these stages they all stain alike. Some of these spheres then become united in 

 a linear series, to form the chromosomes, while the others (a large proportion of the 

 whole number) take no part in the formation of the chromosomes and are finally dis- 

 solved in the nuclear sap, or are transformed into linin threads. Those spherules 

 which enter into the formation of the chromosomes again become solid and stain 

 more deeply than the others (basichromatin) figs. 50, 51, 62, 63, while those which 

 do not form chromosomes stain less deeply with nuclear stains and gradually come 

 to take plasma stains, (oxychromatin.) 



In the prophase of the third, fourth and fifth cleavages the chromatin exists in 

 the form of a reticulum, figs. 70, 71, 74, 75, and not in the form of separate sphe- 

 rules. In the rest preceding the prophase, however, this reticulum is formed of 

 chromatin spherules as in the first and second cleavages, though these spherules are 

 never so evident in later cleavages as in the first two. Some of the threads of this 

 chromatin reticulum become chromosomes ; others which show that they are com- 

 posed of granules, fig. 70, stain much less deeply with nuclear stains, finally taking 

 plasma stains only, and have no part in the formation of chromosomes, but are 

 dissolved in the nuclear sap, or are transformed into linin. 



This differentiation into two kinds of chromatin, one of which (basichromatin) 

 forms chromosomes and the other (oxychromatin) does not, occurs in the early pro- 

 phase ; in the preceding rest stages all the chromatin, both reticulum and spherules, 

 stains alike, figs. 45-55 and 61-64 and 69-70. In the first and second cleavages the 

 oxychromatin granules are scattered through the whole of the nucleus and most of 

 them dissolve in situ, figs. 53, 54, though some of them become attached to the 

 mantle fibres of the spindle, fig. 55, and text figs. XVII and XVIII, wdiere they are 

 either ti-ansformed into spindle fibres or are dissolved, exactly as in the prophase 

 of the first maturation. These dissolving granules sometimes remain hollow and 

 in this case their morphology sufficiently identifies them with the chromatin spher- 

 ules of preceding stages, figs. 49-52 ; in other eggs the dissolving granules become 

 solid and gradually grow smaller and smaller until they disappear in an almost 

 homogeneous nuclear sap, figs. 53 and 63. In some of the cleavages, particularly 

 the second and third, I have observed that the basichromatin, in the form of a 

 densely staining reticulum occupies that portion of the nucleus lying nearest the 

 centrosome ("Pol" of Rabl '85), while the oxychromatin, also in the form of a 

 reticulum, occupies the opposite half of the nucleus (" Gegenpol " of Rabl ), figs. 62 



' Tliese hollow spherules with clear center and dark periphery' directly reverse that coinmon 

 staining phenomenon, such as is characteristic of yolk spheres, where the periphery becomes clear, on 

 destaining, and the center remains dark. They have also been figured by Korschelt ('95) in Oiihryo- 

 trocha and by Coe ('99) in Cerebratulus in the prophase of the first cleavage. 



