EARLY STAGES OF DEVELOPMENT OP THE RABBIT. 139 



If the cells of the inner mass do not multiply quickly enough, 

 and if they are not connected together firmly, and if they 

 adhere by some means or other to the outer layer, then, as 

 the outer layer expands, there must be a tendency for the 

 general separation of the cells of this inner mass, and this 

 will be most apparent at the edges. That is to say, 

 the centre, originally thicker than the edges, will become 

 thinner, and at the periphery the edges will be drawn out 

 to an irregular or regular fringe according to less or more 

 regular local growth and expansion of the outer epiblastic 

 layer. 



If the spread of these is due to this cause, and if the growth 

 is equal over the whole surface of the sphere, then, in a section 

 taken through the centre of the sphere and the centre of the 

 embryonic disc, the arc along which these isolated cells are 

 found should subtend an angle equal to that subtended by 

 the compact embryonic disc up till now ; that is to say, an 

 angle of about 80°. But on measurement at a rather later 

 stage, 102 hours after coition, represented in fig. 39, the 

 angle is found to be very considerably wider than 80°, in fact 

 somewhere between 110° and 130° (the limit of these cells 

 in question being very irregular), and by the eighth day an 

 angle of 200°. 



Hence it would seem that this does not account for the 

 apparent growth round. If, however, we have any reason to 

 suppose that the cells of the outer layer multiply more rapidly 

 and thus allow the expansion of that part of the sphere to take 

 place quicker around a zone bounded roughly by the edges of 

 the compact embryonic area on the one hand and some other 

 parallel line about the original equator on the other hand, we 

 could still consider that the suggested cause is the correct one. 

 I think we have, for these reasons : 



The blastodermic vesicle of 96 hours is a sphere. The 

 blastodermic vesicle of 120 hours is no longer a sphere, but 

 is a body of such a nature that a horizontal plane taken 

 through its longest diameter will not pass through the equator, 

 but will be nearer the upper pole than the lower. 



