GERM-CELL HISTORY IN THE BROOK LAMPREY 3 



The work of Brock ('81), Schreiner ('04), R. Hertwig ('05, 

 '06, '07), and others has shown that in both sexes of certain 

 species of vertebrates there is a tendency toward hermaphrodi- 

 tism at the period of differentiation of the primordial germ cells — 

 an indication that sex is not irrevocably determined at the time 

 of fertilization. This fact and the lack of a complete account 

 of the germ-cell cycle in any vertebrate led me to undertake a 

 study of the whole history of the germ cells in both sexes in the 

 American brook lamprey, Entosphenus wilderi (Gage), in which 

 species there is a decided tendency toward a condition of juve- 

 nile hermaphroditism. This seemed the more worth while 

 because the lampreys and the hag-fishes are now elevated to a 

 separate vertebrate class, and in this class very little work has 

 been done on the germ-cell cycle. 



I wish to express my sincere appreciation of the help and 

 encouragement received from Prof. Jacob Reighard during the 

 progress of the work. 



SPAWNING HABITS AND LIFE-CYCLE OF ENTOSPHENUS WILDERI 



A. Spawning habits 



Entosphenus wilderi is abundant about Ann Arbor in several 

 streams tributary to the Huron River. All the material used 

 in the present study was collected from Honey Creek, a small 

 stream about four miles west of the city. 



On the average, the first lampreys appear on the spawning 

 grounds at Ann Arbor about the 10th of April. Dean and 

 Sumner ('97) report them spawning on the 16th of April in New 

 York City. According to Gage ('93), they spawn between the 

 8th and 20th of May at Cayuga Lake, New York. The time of 

 spawning is undoubtedly dependent upon temperature as deter- 

 mined by the progress of the season. The temperature of the 

 water in Honey Creek, when the lampreys first appear in the 

 spring, ranges from 13°C. to 14^°C. The water is warmer down 

 stream than farther up, and it is usually down stream that the 

 lampreys are first observed. This, of course, may be due to the 



