M0K0GR.4PH OF THE EXISTING CRINOIDS. 379 



appears, but the thread character of the chromatin is always evident. In the reso- 

 Uition of the sjniapsis the nucleolus is seen to be still present, of normal size, 

 deeply stained, and sharply spherical, while the chromatin appears to radiate out 

 from it onto the nuclear membrane in the form of (apparently) eight deeply 

 stained and homogeneous strands. The latter lose their apparent connection with 

 the nucleolus and, without further segmentation, give rise to the (apparently) 

 eight U-shaped chromosomes which leave the nuclear membrane and aggregate 

 together in the center of the nucleus to form the equatorial plate. From this stage 

 onward it is not possible to detect the nucleolus, nor is there any trace of this 

 structure in the completed spindle. 



In spite of the large number of oogonia found in active mitosis, Professor 

 Chubb never saw the anaphase stages of these oogonial divisions, and he was 

 therefore imable to trace the actual formation of the oocyte. 



In what appears, however, to be the stage immediately succeeding the com- 

 pletion of the last oogonial division, the young oocyte possesses a distinct nuclear 

 membrane inclosing a spherical mass measuring 5 \i. in diameter in the center of 

 which the nucleolus is suspended by a faintlj' stained linin reticulum. The nucle- 

 olus measures 1.25 [i in diameter and is. after most fixing reagents, deeply stained: 

 as in the oogonia, it is surroimded by a laj'er of chromatin. In the oocyte, however, 

 though the whole of the chromatin present is massed around the nucleolus, it forms 

 a layer too thin to affect appreciably the spherical form of the latter. 



The absence of anaphase stages has made it impossible to determine the part 

 taken by the massive chromosomes of the oogonium in the formation of the nucleus 

 of the oocyte, and therefore also the mode of origin of the nucleolus. But having 

 regard to the behavior of the nucleolus in the oogonium — the rarity with which 

 it is seen during the formation of the equatorial plate and its invariable absence 

 in the completed spindle — it seems fairly certain that this structure arises in the 

 oocyte as a new formation is not genetically related to that of the oogonium. 



The invariably central position of the rapidly growing nucleolus of the young 

 oocyte conclusively shows that the peripheral position of this structure in the rela- 

 tively quiescent oogonium can not have any significance as denoting an extranuclear 

 origin for this structure. 



The young oocyte grows rapidly, and with the growth of the cell body there 

 is a corresponding growth of the nucleus and nucleolus. Early in this process 

 the nuclear reticulum, at first faintly stained, commences to stain more deeply. 

 This increase in the staining capacity of the nuclear reticulum proceeds from the 

 nucleolus outward, and though the nuclear reticulum can, with suitable illumina- 

 tion, be seen really to extend from the nucleolus out to the nuclear membrane, 

 the process gives it the appearance of consisting of longer or shorter, more or 

 less deeply staining threads, radiating out from the surface of the nucleolus. 

 Sooner or later the basophile reaction has extended over the whole of the nuclear 

 reticulum. Soon after this the strands lose their connection with the surface of 

 the nucleolus and with the nuclear membrane and become irregularly scattered 

 over the nucleus, an arrangement which is maintained throughout the remaining 



