COMPARATIVE ANATOMY. 



Fi. 7, A AND B. DIAGRAMMATIC TRANSVERSE SECTIONS THROUGH A DEVELOPING 

 VERTEBRATE EMBRYO. 



D, alimentary canal ; Ent, hypoblast, showing in Fig. A the thickening which will 

 form the notochord ; Ch l (Fig. B), the notochord now constricted off from the 

 hypoblast; UIV, mesoblastic somite; UG, primary urinary duct (segmental 

 dnct) ; A, aorta ; tipP, splanchnic and SoP, soniritic rnesoblast ; Co, Cod, coelome ; 

 H, remains of the upper part of the coilome in the interior of the mesoblnstic 

 somites ; Ekt, epiblast ; Med, medullary cord : in Fig. A it is shown still con- 

 nected with the epiblast, from which it has become constricted off in Fig. B. 



A large cleft appearing in the mesoblastic tissue divides it into 

 a parietal layer (Fig. 7, A and B, SoP), lying along the inner 



