50 PETER OKKELBERG 



D. Period of sex differentiation 



1 . General statement. The period following that of secondary 

 multiplication of the germ cells is characterized in dioecious ani- 

 mals by the differentiation of male and female individuals. The 

 latter may be distinguished from the former by the appearance 

 in the germ gland of yolk-filled oocytes. This phenomenon is 

 accompanied or preceded by somatic changes in the germ gland 

 and elsewhere. The somatic differentiation may take the form 

 of changes in the structure of the germ gland, the appearance 

 of accessory reproductive organs peculiar to one sex or the other, 

 or the development of other secondary sexual characters. 



In the lamprey there are no secondary sex characters developed 

 until after metamorphosis, before which the sexes cannot be dis- 

 tinguished except by an examination of the germ gland, and in 

 early stages even the germ gland does not form a criterion of the 

 future sex of the animal. 



2. Sex characters in the adult brook lamprey. The reproductive 

 gland in the adult lamprey is unpaired and is suspended by a 

 mesentery from the middorsal line. Previous to spawning, it 

 fills practically the whole body cavity in both sexes. The surfaces 

 of both the male and female glands are thrown into more or 

 less oblique folds, more easily seen in the testis than in the 

 ovary. The testis is made up of numerous cysts filled with 

 spermatozoa and enclosed by follicle cells. It is supported by 

 the mesorchium, from which connective tissue cords radiate into 

 the body of the gland. In the ovary each ovum is enclosed by 

 follicle cells as are the cysts in the testis. Connective-tissue 

 cords, similar to those of the testis, radiate into the body of the 

 gland from the mesovarium above. The number of ova varies 

 in different females as also does their size. In the same animal, 

 however, the ova do not vary greatly in size. 



The cysts in the testis and the ova in the ovary may be con- 

 sidered homologous structures. In the one case the germ cells 

 have continued to divide, while in the other case they have 

 stopped dividing early in the life of the animal and have entered 

 upon a period of growth. The greatest amount of growth takes 

 place in the female after metamorphosis, while in the male 



