B 



504 embryology: frog 



of the blastopore lip. From the walls of the archenteron the 

 endoderm proper (epithelium of midgut, p. 638), notochord, and 

 mesoderm arise as follows. 



At each side the floor grows up within the roof as a sheet 

 which eventually meets its fellow in the mid-dorsal line and so 

 shuts off the original roof from the cavity. The complete lining 

 of the gut which is thus formed from the yolky cells is the endo- 

 derm proper. The layer, now outside it, formed by lip growth, 

 becomes in the mid-dorsal line the notochord and at the sides 



the mesoderm, which lies as a 

 mantle around the endoderm. Be- 

 cause of the way in which the lip 

 which formed it was completed, the 

 mesoderm mantle, which in front 

 is dorsal only, behind completely 

 surrounds the endoderm. By growth 

 of its fore-edge it spreads forwards 

 and downwards between ectoderm 

 and endoderm, forming a layer all 

 over the embryo, except in the mid- 

 dorsal line, where the notochord 

 lies. It will be seen that the 

 mesoderm first arises in the position 

 in which the mesodermal grooves 

 arise in Branchiostoma ; moreover, as 

 in Branchiostoma, the cells which 

 constitute it were from the first 

 distinct from the endoderm and 

 reached their position later, by rolling 

 in over the blastopore lip. On each side of the notochord the meso- 

 derm is thicker than elsewhere, forming the segmental plate. 

 A split, the rudiment of the coelom, appears and separates an 

 outer or somatic layer from an inner or splanchnic layer (Fig. 

 495 A). This split does not extend far into the segmental plate, 

 from which it soon disappears, but it enlarges in the ventral 

 region ; a coelom formed in this way, by splitting of mesoblast, is 

 called a schizocoele. The segmental plates now divide into a 

 series of blocks, the epimeres — usually, though incorrectly, 

 called ' mesoblastic somites ' — separating from the lateral plates, 

 which do not segment. The lateral plates correspond to the 

 fused hypomeres of Branchiostoma (pp. 627-28). The epimeres 



r.«. 



Fig. 494. — A still later embryo. 



A , From behind and above ; B, from in 

 front. 



an., Proctodaeum ; g.a., gill arches ; 

 s., sucker ; s.p., sense plate. 



