CHAP. III.] THE EMBEYOJ^IC SHIELD. 49 



dates of the several changes are, within the Hmits of 

 some hours, very uncertain, particularly in the first few 

 days ; one egg being found, for example, at 36 hours in 

 the same stage as another at 24 or 30 hours, or a third 

 at 40 or 48 hours. "When we speak therefore of any 

 event as taking place at any given hour or part of any 

 given day, we are to be understood as meaning that 

 such an event will generally be found to have taken 

 place at about that time. We introduce exact dates 

 for the convenience of description. 



The changes which take place during the first day 

 will be most easily considered under several periods. 



From the 1st to about the 8th hour.— During this 

 period the blastoderm, when viewed from above, is 

 found to have increased in size. The pellucid area, 

 which at the best is but obscurely marked in the unin- 

 cubated egg, becomes very distinct (the central opacity 

 having disappeared), and contrasts strongly with the 

 opaque area, which has even still more increased both 

 in distinctness and size. 



For the first few hours both the pellucid and opaque 

 areas remain approximately circular, and the most im- 

 p ortant change, besides increase in size and greater 

 distinctness which can be observed in them, is a slight 

 ill-defined opacity or loss of transparency, which makes 

 its appearance in the hinder half of the pellucid area. 

 This is known as the embryonic shield. 



Slight as are the changes which can at this stage be 

 seen from surface views, sections taken from hardened 

 specimens bring to light many most important changes 

 in the nature and arrangement of the constituent 

 cells. 



F. & B. 4 



