VI.] THE INTERMEDIATE CELL-MASS. 189 



day (Fig. 64). The chief differences between them arise from 

 the great increase in the space (now filled with mesoblast-cells) 

 between the notochord and the hypoblast. In addition to this 

 we have in tlic later section the completely formed amnion, the 

 separation of the muscle-plate from the mesoblastic somites, the 

 formation of the Wolffian body, etc. 



The mesoblast including the Wolffian body and the muscle- 

 plate (m.p.) is represented in a purely diagrammatic manner. 

 The amnion, of which only the inner limb or true amnion is 

 represented in the figure, is seen to be composed of epiblast and a 

 layer of mesoblast ; though in contact with the body above the 

 top of the medullary canal, it does not in any way coalesce with 

 it, as might be concluded from the figure. 



formed muscles in erabryo birds have an arrangement 

 like that which is permanent in fishes; being longi- 

 tudinal in direction, and divided into segments. 



The remainder of the somites, after the formation of 

 the muscle-plates, is of very considerable bulk ; the cells 

 of the cortex belonging to them lose their distinctive 

 characters, and their major part becomes converted, in a 

 manner which will be more particularly described in a 

 future chapter, into the bodies of the permanent ver- 

 tebrae. 



We may merely add here that each of these bodies 

 sends a process inwards ventral to the medullary cord, 

 and that the processes from each pair of these bodies 

 envelope between them the notochord. 



The intermediate cell-mass and the Wolffian body. 

 In a transverse section of a 45 hours' embryo a consider- 

 able mass of cells may be seen collected between the meso- 

 blastic somites and the point where the divergence into 

 somatopleure and splanchnopleure begins (Fig. 34, just 

 below W.d). This mass of cells, which we have already 



