2 PETER OKKELBERG 



5. Discussion of the hermaphroditic condition found in the lamprey 



in connection with other sex phenomena not easily explained 



by current theories 88 



a. Normal hermaphroditism 88 



h. Alternation of the hermaphroditic and dioecious condition 90 



c. The effect of delayed fertilization on sex 92 



d. Hermaphroditism and sex reversal due to external conditions 94 



e. Hermaphroditism as a result of hybridization 98 



/. Sporadic hermaphroditism 102 



g. Hermaphroditism as a result of hormone action 103 



h. Hermaphroditism as a result of parasitism 103 



{. Sex in parthenogenetic animals 104 



j. Variation in sex ratios 105 



6. General conclusions in regard to the problem of sex determination 106 



General summary of observations 115 



Conclusions 116 



Literature cited 118 



INTRODUCTION 



Owing to their bearing on current theoretical questions, cer- 

 tain phases of the germ-cell cycle in vertebrates have received 

 more attention than others. Maturation has been fully studied 

 because it appears to involve a redistribution of parental heredi- 

 tary factors. The small size and the brief maturation period 

 of the spermatocytes make it possible to obtain all stages very 

 easily on a smgle slide, whereas much time and material are 

 needed to obtain the stages of maturation in the much larger 

 oocytes. Since it usually is assumed that the maturation phe- 

 nomena are practically identical in the two sexes, the male has 

 been commonly selected for the study of this period because it 

 presents lesser technical difficulties. A second phase on which 

 attention has been focused in recent years, because of its bear- 

 ing on the question of germ-plasm continuity, is that of the 

 origin and early development of the germ cells. The period of 

 differentiation of the male and female sex cells from the pri- 

 mordial germ cells has been relatively neglected, apparently 

 because of the belief that in all animals sex is irrevocably deter- 

 mined at or before fertilization through the agency of sex chro- 

 mosomes. The result is that we have much literature on isolated 

 periods in the germ-cell history of vertebrates, chiefly in the male 

 sex, but few comprehensive accounts. 



