EAELY STAGES OF DEVELOPMENT OF THE RABBIT. 153 



CHAPTER V. 



Changes that occur during the Seventh Day (145th to 

 168th Hours). 



The Fate of the Outer Layer of Epiblast (or 

 Eauber's Layer) in the Embryonic Disc. 



The embryos have now grown to such a size as to cause 

 them to respond more effectually to the impulses set up by 

 contractile movements of the muscular walls of the uterus, 

 and therefore we find them much further advanced along 

 the uterine tube, and more scattered. They have not, how- 

 ever, taken up a permanent position as yet, for although this 

 may occur in some cases during the last few hours of the seventh 

 day, more usually it does not take place until the early part of 

 the eighth day. 



It will be best to describe the course of events in the three 

 layers separately. 



Outer Layer of Epiblast. — Very little need be said of the 

 greater part of this layer, no change except such as has been de- 

 scribed as occurring during the fifth and sixth days takes place. 

 But special attention must be given to that part of the area which 

 lies over the patch of inner layer of epiblast, i. e. embryonic 

 disc. 



Inner Layer of Epiblast. — During the early part of the 

 seventh day the cells of this layer are just as described in 

 the preceding chapter. They extend now over an area of 

 about '6 mm. The general outline of the mass is still circular. 

 Each cell is distinct and rounded, with very large nucleus ; and 

 with nearly all stains that I have used they stain only lightly. 



Early on the seventh day these cells show signs of greatly 

 increased activity. They multiply, become pressed together, 

 and now form a very compact layer at the same time as certain 

 changes occur in the outer layer of epiblast. 



The course taken by these two layers during the next few 

 hours, and its significance, have been very difi'ereutly described 



