GERM-CELL HISTORY IN THE BROOK LAMPREY 5 



able that fertilization of nearly all the eggs is insured. In fact, 

 all eggs collected from the stream after deposition are found to be 

 developing. 



B. Life-cycle 



The life-cycle of the lamprey may be divided into three main 

 periods: the embryonic, the larval, and the adult (table 2). 

 There is no sharp structural change between the first and second 

 periods, but the first period may arbitrarily be considered as 

 ending at the time of hatching and the larval period at the time 

 of metamorphosis. The duration of the embryonic period de- 

 pends on the temperature of the water during development. 

 Balfour ('81) estimated the length of the period to be from 

 thirteen to twenty-one days in Petromyzon planeri. A. Mliller 

 ('56) says that in the European brook lamprey (kleines Neun- 

 auge) hatching takes place about eighteen days after fertiliza- 

 tion. I have found that when the temperature of the water 

 ranges from 17 to 20°C., the artificially fertilized eggs of Ento- 

 sphenus wilderi hatch in the laboratory in about ten to twelve 

 days. There may, however, be a great deal of variation in the 

 rate of development, even in the same brood of eggs when kept 

 in the same dish. 



The length of the larval period is not known from direct 

 observation. It would be difficult in the laboratory, if not 

 impossible, to raise the larvae to the time of metamorphosis, 

 and if one were successful in doing so, he could not be certain 

 that the larval period was of the same length as in the natural 

 habitat. Various estimates have been given. A. Miiller ('56) 

 states that metamorphosis probably does not take place before 

 the fourth year in the case of the European brook lamprey. 

 Loman ('12) found larvae of four sizes, which he took to repre- 

 sent four successive generations, or four years of development 

 before metamorphosis. Lubosch ('03) estimates the adult age 

 to be four years, the estimate being based on the size of the 

 larvae found. According to him, they attain an average size 

 of 5 cm. during the first year; 10, during the second year, and 15 

 to 18 cm. during the third year, or just before metamorphosis. 



