488 CHARLES H. SWIFT 



('92) was able to follow them back directly through all the cleav- 

 age stages to the two-cell stage. 



Haecker ('97), in Cyclops, and Hegner ('09), in some Chry- 

 somelid beetles, were able also to trace the primordial germ-cells 

 back to early segmentation stages. 



It may be said, then, that in many groups of vertebrates, as 

 well as invertebrates, all the primordial germ-cells do not origi- 

 nate in a differentiated portion of the coelomic epithelium called 

 the germinal epithelium. In many of the cases cited the exact 

 origin of the primordial germ-cells has not been found, but even 

 in these it has been definitely established that the germ-cells 

 arise in the early history of the embryo long before the appear- 

 ance of a germinal epithelium. So, with the evidence at hand, 

 it would seem that Nussbaum was correct when in 1880 he form- 

 ulated the hypothesis that ''the sexual cells do not come from 

 any cells that have given up their embryonic character or have 

 gone into building any part of the body, nor do sexual cells ever 

 go into body formation." . 



The bird groups still offer an excellent opportunity for research 

 into the history of the primordial germ-cells, for, notwithstand- 

 ing the amount of work which has been done by Nussbaum, Hoff- 

 mann, Rubaschkin, von Berenberg-Gossler, and others, the ori- 

 gin of the primordial germ-cells still remains ^nrevealed. The 

 stage of 22 or 23 somites is the youngest in which germ-cells 

 have been described among birds. 



In carrying out this research I have received constant aid and 

 advice from Prof. R. R. Bensley. I wish to thank him for this, 

 as well as for a training which has made possible whatever in 

 my results may be considered of value. I also wish to thank 

 Dr. E. V. Cowdry and Dr. G. W. Bartelmez for their assist- 

 ance as well as kindness in allowing me to use their excellent 

 preparations. I am also indebted to Mr. A. B. Streedain for the 

 drawings which illustrate this article. 



