300 CALKINS. 



EXPLANATION OF PLATE XVIII. 



Fig. 9. Spermatogonium in the anaphase of karyokinesis ; chromosomes of 

 the daughter nuclei single and 32 in number (only 19 and 21 are shown in the 

 figure). 



Fig. 10. Resting nucleus of spermatocyte of the first order, showing nucleolus 

 and the reticulated chromatin. 



Fig. n. Spermatocyte with the chromatin in a lump at one part of the cell; 

 first phase after the resting period. 



Fig. 12. Spermatocyte of the first order. The cells of spermatosphere show 

 different stages in the formation of the Knauel or spirem. At D the spirem is 

 double. The archoplasm is in different stages of division. 



Fig. 13. Spermatocyte of the first order, showing double spirem in at most 

 one or two pieces. 



Fig. 14. Spermatocyte of the first order, showing" the spirem breaking into 

 parts. 



Fig. 1 5. Spermatocyte of the first order, showing formation of double chromo- 

 somes and the presence of Nebenkern. 



Fig. 16. Spermatocyte of the first order, showing 32 double chromosomes. 



Figs. 17, 18, 19, 20, and 21, spermatocytes of the first order, showing different 

 stages in the formation of the four-groups, or Vierergruppen. 



Figs. 22, 23, 24, 25, 26, and 27. Different stages in karyokinetic division of the 

 spermatocyte of the first order. 



Figs. 28, 29, and 30. Different aspects of the spindle of spermatocytes of the 

 second order. Fig. 30 represents the nuclear plate of a spermatocyte of the 

 second order, showing 16 double chromosomes. 



Figs. 31, 32, and 33. Spermatids, showing different stages in the concentration 

 of the chromatin. 



