20 EARLY AMPHIBIAN DEVELOPMENT 



of the notochord), continuous with one another posteriorly and 

 ventrally, round the rim of the blastopore. The lateral edge of each 

 sheet of mesoderm rests upon and is more or less confluent with 

 the outer surface of the endoderm of the floor and sides of the gut, 

 and this confluence follows a line passing diagonally forwards and 

 upwards from the ventral lip of the blastopore, on each side of the 

 embryo. The lateral edges of the mesodermal sheets then become 

 free from the endoderm, and gradually extend forwards and 

 ventrally. The mesoderm, which ultimately comes to be situated 

 in the mid-ventral line of the embryo in front of the blastopore, and 

 which among its derivatives will include the heart, is thus of paired 

 origin. That portion of each sheet of mesoderm which immedi- 

 ately flanks the notochord undergoes metameric segmentation to 

 form the somites and myotomes, while the remainder gives rise to 

 the unsegmented mesoderm of the lateral plate. The kidney tubules 

 arise from tissue on the margin between the segmented and un- 

 segmented portions of the mesoderm. 



It will be noticed from this account that in the Urodele, the 

 mesoderm and endoderm are separate zones, more or less sharply 

 marked off from one another, from the very outset of and right 

 through gastrulation. The endoderm is soon fashioned into a cup 

 in the antero-ventral region of the embryo, with its concavity facing 

 backwards and upwards : the mesoderm forms another cup, in the 

 postero-dorsal region of the embryo, inverted over the endoderm 

 cup, and with its concavity facing forwards and downwards. Each 

 of these two cups then completes itself into a hollow sphere by the 

 growth of its margins. In this way, the endoderm undergrows the 

 mesoderm and notochord to form the definitive gut-roof, while the 

 mesoderm overgrows the endoderm until it eventually encircles it 

 almost completely.^ 



^ The detailed study of the processes of gastrulation and germ-layer formation 

 in Urodela and in Anura throws an important light on the distinction (based on 

 morphological considerations) between peristomial and gastral mesoderm. The 

 former is regarded as derived from the active tissue round the rim of the blasto- 

 pore, while the latter is supposed to be derived (by delamination or evagination) 

 from the wall of the gut. In both Urodela and Anura the mesoderm is derived 

 from a ring of tissue surrounding the blastopore, and is, strictly, peristomial. But 

 in Anura the conditions of invagination are such that the mesoderm is rolled in 

 as a mantle closely applied to the endoderm, and it is its subsequent delamination 

 from the latter which gives the mesoderm the appearance of being of gastral 

 origin. 



