THE DEVELOPMENT OF THE ALBINO RAT, 

 MUS NORVEGICUS ALBINUS 



[I. ABNORMAL OVA; END OF THE FIRST TO THE KM) OF THE 



NINTH DAY 



CONTENTS 



Introduction 115 



Half embryos in Mammalia 117 



Degeneration of ova at the end of segment at ion 120 



Incomplete or retarded segmentation 121 



Abnormal segmentation cavity formation 126 



Degeneration of ova as a result of pathologic mucosa 129 



Imperfect development of ectodermal vesicle 132 



Two egg-cylinders in one decidual crypt 138 



Conclusions 140 



Literature cited 142 



INTRODUCTION 



In the course of my study of the normal development of the 

 albino rat, from the end of the first to the end of the ninth day 

 after insemination, as recorded in Part I of this series of contri- 

 butions, there were encountered from time to time ova which 

 appeared to deviate both in rate and type of development from 

 what, as a result of extended study, came to be regarded as the 

 normal developmental cycle of the albino rat. AY hen taken col- 

 lectively, the number of these abnormal ova is not large, al- 

 though they embrace nearly all of the developmental stages 

 studied. When taken singly, it may be stated that while it is 

 comparatively easy to record the points of deviation from the 

 normal, it must be admitted that the probable fate of the respec- 

 tive stages can only be conjectured. Nevertheless, a record of 



115 



