EXPLANATION OF PLATE IV. 



General scheme of the maturation divisions (meiosis) in the spermatogenesis of 

 Lepidosiren. From W. E. AGAR, Quart. Journ. Micr. Soc. Vol. LVII. 1911, 

 p. 9; figures kindly lent by Dr AGAR.) 



A. Resting nucleus, primary spermatocyte. B. Leptotene stage. C. Threads 

 beginning to pair side by side (Lepto-zygotene stage). D. Completion of 

 pairing (" bouquet " stage, zygo-pachynema). E. Strepsitene stage, begin- 

 ning of synizesis. The double threads have split except at their ends, forming 

 long loops. F. Late synizesis. G. End of synizesis. Bivalent loops are 

 shortening and thickening, and are beginning to break up into their con- 

 stituent univalents. H. The nuclear membrane has disappeared. The loops 

 have broken up into the diploid number of chromosomes each with a trans- 

 verse constriction. I. Appearance of the spindle. Second pairing of the 

 chromosomes beginning. J. Second pairing complete ; definitive bivalents 

 taking their place on the spindle. K. Anaphase of first spermatocyte division ; 

 bivalents separated into univalents each still having a transverse constriction. 

 Spindles rotating in preparation for the second division. L. Metaphase of 

 second spermatocyte division. Univalents, still transversely constricted, 

 splitting longitudinally. 



