DEVELOPMENT OF THE ALBINO RAT 



73 



here and there to the inner surface of the parietal ectoderm, in 

 the albino rat at no t inie forming a eont 'unions layer, lie has desig- 

 nated as the parietal entoderm. He is followed in this by Wida- 

 kowich. This nomenclature has been used by me in the sense 

 employed by Sobotta. The parietal or transitory ectoderm 

 i Kolster's 'feinfasserige Haut') forming the roof or antimesome- 

 t rial portion of the vesicles, is constituted of a single layer of 

 flattened cells, which in the rat show no regional differentiation. 

 The resorption of maternal blood, incidentally noted with 

 reference to cells of the ectoplacental cone and certain of the 

 cells of the parietal ectoderm in connection with vesicle ( ' of 

 figure 24, to which phenomenon attention has been drawn by 

 Sobotta and Kolster for the mouse, will receive further consider- 

 ation in the discussion of older stages. 



DEVELOPMENT AND DI1TKRKNTIATION OF THE 

 EGG-CYLINDER 



The material at hand is listed in table S. 



For the stages showing the development and differentiation 

 of the egg-cylinder in the albino rat I am able to present a series 

 of stages which follow one another in close succession. The 

 figures presented are in themselves so elucidative that an extended 

 description is obviated. The stages under consideration fall 

 within the eighth day after the beginning of insemination, 

 judging from the great majority of the specimens at my dis- 

 posal, although two rats (Nos. 17 and 35) killed in the latter 



