10 PETER OKKELBERG 



desired stage. The older larvae collected from the stream 

 were usually anaesthetized and put into the fixing solution as 

 soon as they reached the laboratory. Precaution was taken to 

 insure a rapid fixation of the germ gland. 



Various fixing reagents were used, but the best results were 

 obtained with Flemming's, Meves', and Bouin's solutions. For 

 embryos in which there was a great deal of yolk, Bouin's solu- 

 tion gave very satisfactory results; it also gave uniformly good 

 results for all other stages. For certain nuclear and cyto- 

 plasmic structures, Flemming's and Meves' solutions were more 

 satisfactory. After fixation the material was left in alcohol for 

 a few days and then imbedded. The results were not as good if 

 the material had remained in alcohol for a long time. 



Haemalum with a counterstain of orange G gave the best re- 

 sults for early stages in which a great deal of yolk was present. 

 This combination gave the yolk a yellowish or brownish tint, 

 while the cytoplasm was stained more or less bluish. For later 

 stages iron haematoxylin was used almost exclusively, either alone 

 or with a counterstain of eosin or Licht-griin. 



HISTORY OF THE GERM CELLS 



A. General outline of the whole germ-cell history in animals; spec'al 

 outline for Entosphenus wilderi 



In table 1 an outline is given of the different periods, as 

 defined by various writers, in the development of the germ 

 cells of vertebrates. The scheme recognizes, in the column 

 headed 'periods in the germ-cell cycle,' an early segregation of 

 the germ ceUs and the development of all the definitive germ 

 cells from the early segregated cells. The table also admits the 

 possibility of two alternatives as to the origin of sex in the 

 young animal. There may be distinct male and female indi- 

 viduals from the beginning of embryonic development, as shown 

 in columns I and II of the table, in which case sex is dependent 

 on an hereditary factor or on other factors that influence the 

 germ cells at or before fertilization. On the other hand, the 

 young animal may be indifferent as to sex, as shown in column III 



