37 2 ARTHUR C. WALTON. 



EXPLANATION OF PLATES. 



All figures were made with the aid of a camera lucida, at a magnification of 

 2,800 diameters; Spencer apochromatic objective 2 mm., compensating ocular 

 20 X and projection distance 310 mm. 



FIG. i. Prophase of first spermatocyte, showing formation of chromosomes 

 and separate grouping of heterochromosomes. The plasmosome is just above 

 heterochromosome group. 



FIG. 2. Late prophase, first spermatocyte, spindle formed but nuclear mem- 

 brane still intact. 



FIG. 3. Metaphase plate, first spermatocyte, showing heterochromosome 

 group in center; number of chromosomes, 18. 



FIG. 4. Metaphase plate, first spermatocyte, lateral view showing breaking 

 down of nuclear membrane. 



FIG. 5. Early anaphase, first spermatocyte, optical section through equatorial 

 plate, showing lagging heterochromosome group. 



FIG. 6. Late anaphase, first spermatocyte, showing lagging heterochromosome 

 group of six components. 



FIG. 7. Late anaphase, first spermatocyte, heterochromosome group going to 

 upper daughter cell. Central spindle degenerated. 



FIG. 8a. Polar view of daughter plate of first division receiving heterochromo- 

 some group; number of chromosomes, 18. 



FIG. 8b. Polar view of daughter plate of first division not receiving hetero- 

 chromosome group; number of chromosomes, 12. 



FIG. g. Metaphase plates of two sister second spermatocytes, which have 

 not separated. One plate contains 18, the other 12 chromosomes. 



FIG. ioa. Metaphase plate, second spermatocyte, lateral view of plate con- 

 taining 18 chromosomes, i. e., containing heterochromosome group of 6 and 12 

 autosomes. 



FIG. lob. Metaphase plate, second spermatocyte, lateral view of plate con- 

 taining 12 chromosomes, i. e., lacking the heterochromosome group of six. 



FIG. i id. Telophase of second spermatocyte division, having 18 chromosomes. 



FIG. nb. Telophase of second spermatocyte division, having 12 chromosomes. 



FIG. I2a. Spermatid having 18 chromosomes. 



FIG. I2&. Spermatid having 12 chromosomes. 



