162 ROBERT T. HANCE 



at Glenolden, Pa. It is a pleasure to ackDowledge the kindness 

 and the interest of these various persons and their assistance in 

 helping me get the material I needed. Such friendly codperation 

 between the biological manufacturer, commercial establishment 

 and the theoretical laboratory cannot fail, in the long run, to be 

 productive of results beneficial to all concerned. 



The many facilities placed at my disposal by Dr. McClung, 

 and his constant interest and friendly advice and criticism have 

 made my stay at the University of Pennsylvania not only profita- 

 ble, but very enjoyable, a period always to be remembered with 

 great pleasure. 



I feel that a list of acknowledgments would be incomplete if 

 no mention were made of my appreciation of and indebtedness 

 to my mother and father to whose encouragement, constant 

 interest and sympathy I owe so much. 



THE CHROMOSOMES OF THE GERM CELLS 



The diploid chromosome number 



Only those cells have been considered in which the chromo- 

 somes were well separated (figs. 1 to 20) where the count could 

 be niade with comparative ease and accuracy. Forty chromo- 

 somes have been found constantly in the spermatogonia of the 

 pig. This number has appeared in testicular material from 

 three Berkshires, five Jersey reds and one Poland china, the 

 material being gathered from rather widely separated regions of 

 this country. The drawings in figures 1 to 20, though selected 

 for flatness to show the full length of the chromosomes are not 

 exceptional as to clearness. The photomicrographs in figures 88 

 to 99 emphasize the separation and sharp outlines of the chromo- 

 somes. 



It may occur to the reader that the chromosome number may 

 be influenced by precocious division, since, in the drawings, 

 splits are shown in the chromosomes while they lie in the meta- 

 phase plate. With experience the chromosomes which have di- 

 vided are distinguishable by the. difference of level and diameter. 

 Furthermore, the count of forty has been made in cells in pro- 



