240 THE CHICK. 



of the original lower layer are left, lying between the epiblast 

 and hypoblast, on the establishment of the latter as a distinct 

 and continuous membrane ; and these cells become mesoblast 

 cells (Fig. 108, M). 



(ii) In the middle and lateral portions of the area pellucida. 

 about the time of appearance of the primitive streak, mesoblast 

 cells are budded off freely from the upper surface of the hypo- 

 blast, and form a layer between the epiblast and hypoblast in 

 this region. 



(iii) The horizontal sheets of cells (Fig. 108, PS), which 

 spread out right and left as the wing-like expansions of the 

 primitive streak, and which, it will be remembered, are of 

 epiblastic origin, also take part in the formation of the meso- 

 blast. 



As regards the cells themselves, those of groups (i) and (ii) 

 agree with one another in. being usually of an irregular stellate 

 shape (Fig. 108, M), and in being very loosely arranged. The 

 origin of these two groups is very similar, though not identical ; 

 the cells of the first group being derived from the lower-layer 

 cells, formed by segmentation of the germinal disc ; while those 

 of the second group arise directly from the hypoblast, after 

 this is established as a distinct cellular membrane. It is not 

 possible to draw a sharp line between the two groups, nor to 

 determine in all cases to which group a given cell belongs. 

 Speaking generally, the mesoblast of the body of the embryo 

 itself is derived from group (ii), the cells of group (i) lying almost 

 entirely in the extra-embryonic parts of the blastoderm. 



The cells of group (iii) are derived directly from the epi- 

 blast, and are therefore of totally different origin to those of 

 groups (i) and (ii). They also differ from these latter in their 

 spherical form and more compact arrangement. They are at 

 first (Fig. 108, PS) sharply marked off from the cells of groups (i) 

 and (ii), but as the primitive streak spreads laterally, the cells 

 composing it come into close relation with those of the other 

 groups, and becoming at the same time less compactly arranged, 

 and less regular in form, can no longer be distinguished from 

 those of groups (i) and (ii). The cells of group (iii), or primitive 

 streak mesoblast cells, lie almost entirely behind the embryo, 

 and take but little share in its formation. 



The mesoblast cells of all three groups soon become con- 



