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PHYLUM CHORDATA 



177 



of the lower layer, are the beginning of the definite endoderm. 

 The edge of the in-folding, entitled the embryonic rim, is obviously 

 the equivalent of the dorsal lip of the blastopore in Amphioxus. 

 The endoderm and its underlying cavity soon grow forwards 

 towards the segmentation- cavity. Under the latter appears a 

 floor of lower-layer cells, but the cavity soon becomes obliterated 

 as the archenteron develops. 



FIG. 850. Longitudinal section through the blastoderm of a Pristiurus embryo before the 

 medullary groove has become formed, showing the beginning of the process of in-folding or 

 invagination. al. archenteron ; ep. ectoderm ; er. embryonic rim ; m. mesoderm. (After 

 Balfour.) 



After the formation of the embryonic rim a shield-like embryonic 

 area is distinguishable in front of it, with two folds bounding a 

 groove the medullary groove. The mesoderm becomes estab- 

 lished at about the same time. It is formed from two separate 

 and distinct sources (Fig. 851). Along the edge of the embryonic 

 rim appears a horizontal groove-like depression : this the external 



c.L- 



TVC 



FIG. 851. Pristiums, transverse section of blastoderm, showing the formation of the meso- 

 derm. bp. 1. dorsal lip of blastopore ; c. 6. 1 external coelomic bay ; c. b. z internal coelomic 

 bay ; ec. ectoderm ; en. endoderm ; m. f. medullary fold ; m. gr. medullary groove ; TOS.I 

 external rudiment of mesoderm ; ms.* internal rudiment of mesoderm ; nc. notochord ; 

 yk. yolk ; yk. n. yolk nuclei. (From 0. Hertwig, after Rabl.) 



coelomic bay (c.b. 1 ) marks the line of origin of the peripheral part 

 of the mesoderm (m.s. 1 ), which grows inwards from it as a plate 

 of cells between the ectoderm and the endoderm. The central 

 part of the embryonal mesoderm (m.s. 2 ) is developed from the 

 endoderm at a point immediately external to the rudiment of the 

 notochord : here also a slight groove the internal coelomic bay 



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