MARY T. HARMAX AND FRANK P. ROOT. 



EXPLANATION OF PLATES. 



All the drawings were made with the aid of a camera lucida, a 1.9 oil immersion 

 objective and a number 10 ocular at table level. Then they were enlarged two 

 diameters. The reproductions were reduced ^ from the original. 



PLATE I. 



FIG. I. Spireme of spermatogonial division. 



FIG. 2. Metaphase plate polar view of spermatogonial division showing nine- 

 teen pairs of chromosomes. The pair numbered 19 is the XY-pair. 



FIG. 3. Formation of the primary spermatocyte chromosomes. 



FIG. 4. Metaphase plate of primary spermatocyte chromosome showing the 

 unequal XY-tetrad. 



FIG. 5. Formation of primary spermatocyte chromosomes. This is a later 

 stage than Fig. 3. 



FIG. 6. Metaphase plate of primary spermatocyte chromosomes. 



