author's abstract of this paper issued 

 bt the bibliographic service, december 6 



THE EARLY HISTORY OF THE GERM CELLS IN THE 

 BROOK LAMPREY, ENTOSPHENUS WILDERI (GAGE), 

 UP TO AND INCLUDING THE PERIOD OF SEX 

 DIFFERENTIATION 



PETER OKKELBERG 



Zoological Laboratory , University of Michigan 



FOUR TEXT FIGURES AND TWELVE PLATES (SEVENTY-EIGHT FIGURES) 



CONTENTS 



Introduction 2 



Spawning habits and life-cycle of Entosphenus wilderi 3 



A. Spawning habits 3 



B. Life-cycle 5 



Materials and methods 9 



History of the germ cells 10 



A. General outline of the whole germ-cell history in animals; special 



outline for Entosphenus wilderi 10 



B. Origin and early history of the germ cells 13 



1. Observations on Entosphenus wilderi 13 



2. Historical and critical 22 



3. Discussion 33 



4. Conclusions and summary 39 



C. Period of secondary division 41 



1. Observations on Entosphenus wilderi 41 



2. Historical review and theoretical discussion 45 



3. Summary of the period of secondary division 49 



D. Period of sex differentiation 50 



1. General statement 50 



2. Sex characters in the adult brook lamprey 50 



3. Changes in the germ gland during the period of sex differentiation 51 



a. Changes during mitosis 52 



h. The synapsis phase of the oocytes 55 



c. History and fate of the germ cells during the period of sex 



differentiation 67 



d. Literature on the period of sex differentiation in the lamprey 72 



e. Other cases of juvenile hermaphroditism among vertebrates 73 

 /. Discussion 78 



4. Present status of the sex problem 84 



1 



