GERM-CELL HISTORY IN THE BROOK LAMPREY 51 



metamorphosis is followed by a period of very rapid multiplica- 

 tion of the germ cells. In both sexes the germ cells reach the 

 outside through abdominal pores. These open in each sex into 

 a common urogenital sinus which terminates externally in a 

 urogenital papilla. 



There are only a few external sex-distinguishing characters. 

 The urogenital papilla is short and wide in the female, with a 

 large opening at the end, while in the male it is long and slender 

 with a small opening at the tip (fig. 66). Both sexes have a 

 lateral fold of skin on each side of the papilla. In the female the 

 papilla is hidden by these folds, but in the male it extends beyond 

 the folds as a prominent structure. In the female a small anal 

 fin, connected with the caudal fin by a low ridge of skin, lies 

 directly behind the urogenital papilla, while in the male the anal 

 fin is absent or rudimentary. Loman ('12) states that in the 

 European brook lamprey the anterior dorsal fin is always lower 

 in the female than in the male, and Gage ('93) found the cranial 

 end of the second dorsal fin always swollen in the female of the 

 American brook lamprey. 



From the above description it will be seen that the reproductive 

 organs in the lamprey are reduced to extreme simplicity. The 

 sex glands and the accessory structures, by means of which the 

 reproductive elements are extruded, are morphologically similar 

 in the two sexes. This point is emphasized because of its bearing 

 on the tendency of the animal toward hermaphroditism. 



3. Changes in the germ gland during the period of sex differen- 

 tiation. In larvae less than 35 mm. in length the majority of 

 the germ glands are in an Indifferent condition as regards sex. 

 During these stages the germ cells are scattered irregularly 

 through the germ gland, either singly or in cysts; either in a 

 resting condition or in various phases of mitosis, but they are all 

 alike. This indifferent period is followed by one in which distinct 

 sex characters appear in the germ glands through the develop- 

 ment of oocytes found both in the synaptic and growth phases 

 (tables 1 and 2). Whether the germ glands eventually become 

 ovaries or testes, they all develop this female character, and the 

 animals may therefore during this stage be considered interme- 

 diate as to sex or hermaphroditic. 



