588 ON THE EARLY STAGES IN THE DEVELOPMENT OP THE EMU, 



in tlie former the hypoblast is present as a distinct layer in the 

 hinder part of the area pellucida before the formation of the 

 primitive streak, and it is this, apparently, that has given rise to 

 the view so widely maintained that the mesoblast in the region of 

 the primitive streak is mainly derived from the epiblast, or that 

 the whole mesoblast is so derived. 



During the ensuing few hours the blastoderm increases rapidly 

 in size, its diameter nearly doubling itself in a comparatively short 

 time ; the area pellucida, however, does not increase in dimensions 

 in the same proportion. In a specimen which had been incubated 

 for seventy-eight hours, the area-pellucida (plate IX., fig. 3) was 

 still only about four millimetres in length. A little in front of 

 the middle is a rounded elevation, the head elevation, which slopes 

 away gradually behind, but in front is circumscribed by a well- 

 defined strongly convex border — the border of the head-fold. 

 Running along the axis of this elevation is a narrow and deep 

 fissure, which reaches from close to the convex anterior border to 

 about the middle of the area pellucida, ending apparently abruptlv 

 at both ends. This fissure — the medullary groove — is bounded by 

 a pair of low rounded medullaiy folds which deci-ease in height 

 gradually behind. A little distance behind its posterior end is the 

 beginning of a second longitudinal fissure, the primitive groove, 

 which appears to begin in front in a slight enlargement, but loses 

 itself insensibly behind. 



In the region in front of the head-fold the blastoderm still 

 consists only of two layers of cells — the epiblast and a single layer 

 of irregular amoeboid cells — the mesoblast not appearing in this 

 region till somewhat later. Immediately in front of the head-fold 

 the hypoblast takes on its definite flattened character, and in the 

 head-fold itself the mesoblast makes its appearance and extends a 

 considerable distance outwards. The head-fold has been carried 

 sufiiciently far back to have resulted in the formation of a short 

 rudiment of the[fore-gut (plate XIII. figs. 15-17.). The notochord is 

 distinguishable through a few sections only (plate XIII. fig. 18) as 

 a, median rounded group of cells, having exactly the character of 



