148 



RICHARD ASSHETON. 



as are the cells of the outer epiblast and hypoblast, where they 

 are more intimately connected one with another. In the latter 

 case, that of the hypoblast, cells with a tendency to the features 

 characteristic of both layers of epiblast are to be found. 



CHAPTER IV. 



Changes that occur during the Sixth Day (120th to 

 144th Hours). 



If we may call any period of the development of an animal 

 unimportant, it is to the period between the 1 20th and 144th 

 hours of the development of the rabbit that this epithet might 

 be applied. 



During this day the blastodermic vesicle increases very 

 greatly in size, and assumes very markedly the shape described 

 in the last chapter as being characteristic of the later stages of 

 the development of the vesicle, prior to its attachment to the 

 walls of the uterus. 



The blastodermic vesicle is no longer a sphere. It will be 

 found by measurement to have longer and shorter equatorial 

 axes, and a polar axis which is of less length than the shortest 

 equatorial axis. 



The following measurements are taken from specimens of the 

 earlier part of the sixth day : 



(1) Hypoblast of the Embryonic Disc. — This is now a 

 continuous layer, sufficiently so as to show lines of demarcation 

 between the cells when treated with silver nitrate. This con- 

 tinuous membrane extends a short distance beyond the peri- 

 phery of the inner epiblast layer. The cells composing this 

 membrane are completely flattened. 



(2) Hypoblast beyond the Embryonic Disc. — The 



