34 DEVELOPMENT AND GROWTH OF 



silver nitrate shew that they are much expanded horizontally, 

 but very irregular as to size, varying even within a small area 

 from ffafi ^J^ of an inch in the longest diameter. 



At about the twelfth hour they are uniformly smaller a 

 short way from each extremity of its longer axis than over 

 the rest of the blastoderm. 



It is, perhaps, fair to conclude from this that growth is 

 most rapid at these parts. 



At this time the hypoblast, both in sections and from a 

 surface view after treatment with silver nitrate, appears to 

 end abruptly against the white yolk. The surface view also 

 shews that its cells are still filled with highly refractive globules, 

 making it difficult to see the nucleus. In some cases I thought 

 that I could (fig. 3, a) make out that it was hour-glass shaped, 

 and some cells certainly contain two nuclei. Some of the cells 

 3. b} shew re-entrant curves, which prove that they have 



undergone division. 



The cells of the epiblast, up to the thirteenth hour, have 

 chiefly undergone change in becoming smaller. 



In surface views they are about ^W of an inch in diameter 

 over the centre of the pellucid area, and increase to yoVo f 

 an inch over the opaque area. 



In the centre of the pellucid area the form of the epiblast 

 cells is more elongated vertically and over the opaque area more 

 flattened than was the case with the original upper layer cells. 

 In the centre the epiblast is two or three cells deep. 



Before going on to the further changes of the blastodermic 

 cells it will be well to say a few words in reference to the origin 

 of the mesoblast. 



From the description given above it will be clear that in 

 the chick the mesoblast has an independent origin ; it can 

 be said neither to originate from the epiblast nor from the 

 hypoblast. It is formed coincidently with the latter out of 

 apparently similar segmentation cells. The hypoblast, as has 

 been long known, shews in the chick no trace of its primitive 

 method of formation by involution, neither does the mesoblast 

 shew any signs of its primitive mode of formation. In so 

 excessively highly differentiated a type as birds we could hardly 

 expect to find, and certainly do not find, any traces of the 



