2l6 MARY T. HARMAN. 



(c) Chromosomes of BC. The hybrid BC (Figs. 6-8) has only 

 one hooked chromosome, while its homologue is merely pointed 

 like in CC. This difference is constant in the many metaphase 

 plates examined. There is some variation in the shape of the 

 chromosomes and the size of the bend of the pointed end. In 

 Fig. 6 the hooked member of the third pair is almost crescent 

 shaped, while in the other cells the hooked chromosome is thick 

 at the distal end, tapers toward the proximal end, and then 



bends abruptly. 



2. Growth Period. 



Figures 13 to 24 illustrate the qhanges which take place in 

 the growth period. At the end of the last spermatogonial divi- 

 sions the chromosomes have a crinkled appearance. Figure 13 

 shows all thirteen of the chromosomes in this condition. How- 

 ever the X-chromosome is more compact than the others. This 

 crinkled appearance is due to the chromatin becoming less 

 compact and more granular. There is no sign of a nuclear 

 membrane nor any indication of a separation of the nuclear 

 area from the cytoplasm. Soon the chromatin becomes very 

 finely granular and the chromosomes unite end to end (Fig. 14). 

 Shortly after this stage the chromatin has the appearance of a 

 finely granular continuous thread and a light area separates the 

 cytoplasm from the nuclear material (Figs. 15 and 16). The 

 cell begins to increase in size and the chromatin thread becomes 

 contracted into a ball (Fig. 17). There seems to be no definite 

 polarization to the loops of the spireme, but it has more the 

 appearance of a tangled mass. Sometimes it is with difficulty 

 that the individual fibres are distinguished. Now both the 

 nucleus and the entire cell increase greatly in size (Figs. 18-21) 

 and the chromatin material forms into a thicker thread (Fig. 18). 

 Then the chromatin thread becomes looser and the granules are 

 more concentrated, particularly in places throughout the strand 

 (Fig. 19). The portion of chromatin marked X never becomes 

 as granular as the remainder of it. This chromatin is the sex- 

 chromosome and can be identified in all the stages. 



Following the opening up of the tangled knot the chromatin 

 thread breaks into pieces (Figs. 20 and 21). Figure 22 shows 

 thirteen pieces, twelve of which are uniting in pairs by an end 



