STUDIES ON SEX IN CREPIDULA 35 



Figure 52 shows subseauent stages. The originallv thin threads 

 which went into synizesis become thickened after emerging and 

 form the so-called 'pachytene' nucleus, which is not shown in 

 figure 52, but which soon assumes the appearance of the nucleus 

 in the extreme lower left-hand corner of the figure; i.e., the 

 pachytene threads (which are plainly double) become shortened 

 and thickened. Some of them are always oriented radially to 

 the nucleolus, which at this time is undergoing marked growth. 

 They then lose their sharp outline (fig. 52, upper left-hand cor- 

 ner) and their chromatin begins to distribute itself upon a retic- 

 ulum, while the cytoplasm of the cell increases in volume. 

 Thus the oocyte acquires the resting condition of the nucleus 

 and enters upon the growth period. 



During the formation of the resting oocyte the frequency of 

 primordial male cells becomes less and less, and they are not 

 seen dividing. The follicle nuclei become larger than before 

 (figure 52 is drawn at the same magnification as figures 49, 50, 

 and 51) so that they sometimes become hard to distinguish 

 from the few primordial male cells which remain. The follicle 

 nuclei can usually be identified by their darker stain and the 

 disposition of their chromatin upon a coarser reticulum. 



The oocytes continue to increase in volume uatil they take up 

 nearly all the space of the gonad and cause it to enlarge. When 

 ready for laying the mature ova are so large that the ovary takes 

 up a large portion of the visceral sac. During the later growth 

 stages of the large oocytes the nuclear figures characteristic of 

 the younger stages (synizesis, etc.) become fewer and fewer. 

 Primordial egg cells may always be found lying outside the 

 oocytes next the connective' tissue sheath; the primordial sperm 

 cells disappear entirely. 



History of the accessory reproductive organs 



The undifferentiated goniduct. As has been stated previously, 

 the primordium of the goniduct in postlarval specimens of Crepi- 

 dula plana is a solid strand of cells which extends forward and 

 to the right from a narrowed region of the gonad to the right 



