l8 EARLY AMPHIBIAN DEVELOPMENT 



and ingrowth begins and continues with the greatest activity, the 

 mass of heavy and inert yolk-cells becomes piled up on the opposite 

 or ventral side of the gut-cavity. This alters the position of the 

 embryo's centre of gravity, and as a result the entire embryo rotates 

 ventralwards through about ioo°, until the original egg-axis of 

 polarity is nearly horizontal, and the animal pole faces forwards 

 and a little downwards. The blastopore becomes smaller and 

 eventually closes by the apposition of its lateral lips to one another. 

 At closure, it is situated close to the original vegetative pole, which 

 in its turn, as a result of the embryo's rotation, is now facing back- 

 wards and slightly upwards. 



Internally, meanwhile, the endoderm and the mesoderm are 

 becoming sorted out, so that gastrulation results in the delimitation 

 of the primary germ-layers, ectoderm on the outside, endoderm 

 lining the gut-cavity, and mesoderm in between. 



§3 



The details of the manner in which the mesoderm and endoderm 

 arise in Amphibia have only recently been made out and established, 

 thanks to the method of marking definite regions of the living 

 embryo with easily visible stains, and following them through 

 development.^ 



The following account applies to the Urodele type. The material 

 which becomes tucked and rolled in over the rim of the blastopore 

 on the dorsal side of the embryo, and thus forms the primitive gut- 

 roof, will ultimately give rise to the notochord and some of the 

 mesoderm. Meanwhile, the yolk-laden cells of the original vegeta- 

 tive pole are carried in under the lip of the blastopore by the pro- 

 cess of invagination and find themselves forming the anterior end, 

 floor, and sides of the gut-cavity. Later, these sides grow up be- 

 neath the primitive gut-roof and meet one another in the mid- 

 dorsal line, forming the definitive gut-roof. The remainder of the 

 mesoderm is formed from the material rolled in at the lateral and 

 ventral lips of the blastopore ; though continuous dorsally with the 

 primitive gut-roof, it is never in direct contact with the archenteric 

 cavity (fig. 4). 



The mesoderm thus forms paired sheets of tissue (right and left 

 ^ Vogt, 1929. 



