DEVELOPMENT OF THE PIG DURING FIEST TEN DAYS. 339 



vesicle, which soon after becomes established. If this is so, the 

 embryo in this particular behaves like that of the rat (Robinson 

 [23], Duval [12], or Tupaia javanica, Hubrecht [17]), 

 whereas in forms like the rabbit and mole the blastodermic 

 vesicle cavity arises as a clean cleft between the several seg- 

 m^ents, which are always more sharply separated than in the 

 case of the former animals. In some instances I have seen 

 strands of protoplasm connecting the inner mass with the outer 

 wall of the ab-embryonic pole after the cavity has attained as 

 great a size as in fig. 16. 



Small spherical globules can often be found within the 

 cavity which have the appearance of being oil drops liberated 

 during the process of vacuolation that has resulted in the 

 formation of the cavity of the blastodermic vesicle. 



In fig. 15 the cavity is seen distinctly in two places. In 

 fig. 16 the cavity is large and eccentrically placed. The 

 embryo may be described as a spherical hollow ball, the wall 

 of which is much thickened at one spot. There is as yet no 

 distinct inner mass clearly divided from the outer wall. 



As the embryo grows and the blastodermic vesicle cavity 

 enlarges, the innermost of the nuclei of the thickened portion 

 of the wall become, with their surrounding protoplasm, more 

 and more free and more rounded ; while the outer layer, which 

 is more directly connected with the remainder of the wall, is 

 found to be clearly separated oS from the former, and it 

 becomes alone directly continuous with the outer wall of the 

 vesicle (compare figs. 17 and 18). These changes occur 

 very rapidly, and without much increase in the number of 

 cells. 



The oily yolk globules are still present in little groups in 

 the meshes of protoplasm, chiefly round the nuclei. The 

 whole protoplasm is very much vacuolated, and retains this 

 condition until after the rupture of the zona radiata. 



The inner mass consists now of very few cells. In one 

 specimen, in size between those shown in figs. 18 and 19, the 

 inner mass was completely flattened up against the outer wall, 

 so that the embryonic pole of the blastocyst was not more 



