Manchester Memoirs, Vol. I. (1906), No. 3. 13 



rod of these cells becomes gradually distinct from two 

 lateral sheets {Fig. 4, B) ; this strip is the notochord, the 

 lateral sheets are the mesoderm. Further both notochord 

 and mesoderm become gradually separated from the 

 innermost cell layer, the mesoderm rather earlier than 

 the notochord (which led Balfour to speak of the hypo- 



FlG. 4. 



Differentiation of the roof of the archenteron into notochord, mesoderm, 

 and roof of the definitive gut in the Frog. 

 A, B, C. Three successive stages. 



A. The roof of the archenteron consists of about four layers of cells. 



B. The lowermost layer is distinct from the upper layers at the sides, 

 but not in the middle line {n.ch.). 



C. Both notochord (//.c/;.) and mesoderm (w<?j.) are distinct from the 

 roof of the gut and from one another. 



(Original.) 



blastic origin of the latter) {Fig. 4, C). This inmost 

 layer persists as the definitive roof of the gut, and may, 

 with the yolk-cells which form the floor, be termed the 

 endoderm. 



