THE DEVELOPMENT OF THE ALBINO RAT, 

 MUS NORVEGICUS ALBINUS 



I. FROM THE PRONUCLEAR STAGE TO THE STAGE OF MESO- ' 



DERM ANLAGE; END OF THE FIRST TO THE END 



OF THE NINTH DAY 



CONTENTS 



Introduction 3 



Material and methods 5 



Ovulation, maturation, and fertilization ; 9 



Pronuclear stage 13 



Segmentation stages 21 



2-cell stage 21 



4-cell stage '. 29 



8-cell stage 31 



12 to 16-cell stage 35 



Summary of segmentation stages, rate, and volume changes 36 



Completion of segmentation and blastodermic vesicle formation 42 



Blastodermic vesicle, blastocyst, or germ vesicle 56 



Late stages of blastodermic vesicle, beginning of entypy of germ layers. ... 63 



Development and differentiation of the egg-cylinder 73 



Late stages in egg-cylinder differentiation, and the anlage of the mesoderm 92 



Conclusions 108 



Literature cited 112 



INTRODUCTION 



The early developmental stages of placental mammals, em- 

 bracing the stages of sex cell maturation and fertilization, of 

 segmentation, of blastodermic vesicle and germ layer forma- 

 tion, though subject 'of numerous contributions extending over 

 many years, have in no form been completely investigated. The 

 literature dealing with the phenomena of maturation and fer- 



3 



