356 THE EABBIT. 



cells, cc, formed from the smaller, outer cells of the previoas 

 stages, to the inner surface of which is attached at one pole 

 the mass of inner cells, CD. This mass of inner cells is now 

 flattened out into a lenticular shape ; thicker and more compact 

 in the middle, where the cells are two or three deep, and poly- 

 gonal from mutual pressure ; and thinning towards its margins, 

 where the cells are in a single layer, less closely apposed to one 

 another, and irregular or even amoeboid in form. The vitelline 

 membrane is no longer recognisable : the zona radiata is still 

 present, but, like the outer albuminous investment, is greatly 

 reduced in thickness. 



During the fifth and sixth days, the blastodermic vesicle 

 remains spherical or nearly so in shape, and continues to in- 

 crease rapidly in size. By the end of the fifth day it measures 

 about r5 mm. in diameter ; and by the end of the sixth day, 3 

 to 3'5 mm. 



On the seventh day it becomes ellipsoidal in shape, and by 

 the end of this day (Fig. 143) it measures from 4'5 to 5 mm. in 

 length by 3*5 to 4 mm. in width. Up to the end of the seventh 

 day the several blastodermic vesicles lie quite freely within the 

 uterus, but become gradually spaced out along this, and take 

 up the positions they will retain during the remainder of their 

 development. 



The measurements given above must be regarded as ap- 

 proximate only ; the several blastodermic vesicles in the same 

 uterus vary within certain limits, those lowest down being the 

 largest and most advanced in development ; and one or two at 

 the proximal end of the uterus, nearest to the oviduct, being 

 almost invariably smaller and less developed than the others. 



3. The Germinal layers. 



During the fifth, sixth, and seventh days, important changes 

 occur in the structure of the wall of the blastodermic vesicle, 

 leading to the establishment of the three germinal layers, 

 epiblast, hypoblast, and mesoblast, from which the several parts 

 of the embryo are formed. 



These changes more especially concern the part of the wall 

 of the vesicle to which the lenticular mass of inner cells (Fig. 

 140, cd) is attached ; and to this part, which at" the end of the 

 fourth day is the only portion in which the wall of the vesicle is 



