256 COMPARATIVE ANATOMY 



retarded by the presence of yolk-cells, and we find accordingly 

 that parts of the embryo, which in Amphioxus do not make 

 their appearance till invagination is completed, are formed in 

 the frog while invagination is still in progress. Thus the 

 mesoblast, which in Amphioxus arose from outgrowths of the 

 archenteron, is formed in the frog at an early stage by a 

 splitting of the lower layer into a hypoblastic and a mesoblastic 

 sheet, and the mesoblast sheet is extended in the region of the 

 yolk-cells by the additon to its edges of a number of irregular 

 branched cells derived from the sides and ventral lip of the 

 blastopore, as is shown in fig. 59, K. The neural folds have 

 also made their appearance at and even before the stage repre- 

 sented in fig. 58,y! It is important that the situation of the 

 neural folds should be clearly understood. At one time they 

 were supposed to be formed on the upper or pigmented 

 hemisphere of the egg, but they may more correctly be de- 

 scribed as being formed on the lower or white hemisphere on 

 either side of the line along which the dorsal lip of the blasto- 

 pore has travelled, and this in spite of the fact that the neural 

 folds are always uppermost in the developing egg. Let us see 

 how this has come about. In stage G, fig. 58, the egg has 

 already begun to rotate in a reverse direction to the path of 

 the dorsal lip of the blastopore, and in stage H fig. 58, this 

 rotational movement has continued further, so that the whole 

 egg has moved about a horizontal axis through an angle of 1 10 

 more or less. The reason of this rotation is obvious. The 

 blastocce.le is a cavity situated excentrically in the egg, and as 

 long as it was spacious the hemisphere in which it was con- 

 tained was the lighter and floated uppermost. As the enteron 

 is formed, the blastoccelic cavity is pushed to one side and 

 eventually obliterated, and the new enteric cavity being also 

 excentric, the hemisphere in which it lies is the lighter, so the 

 egg rotates until that hemisphere floats uppermost, bringing 

 the blastopore back to the equator. This final position of the 

 blastopore marks the hind end of the embryo, A meridian 

 drawn from this point through the upper pole indicates the 

 longitudinal axis of the embryo, and this is the path along which 

 the dorsal lip of the blastopore travelled. The epiblast on either 

 side of this meridian is thickened, but becomes thinner 

 along the meridian itself. These changes are expressed in 

 an external view of the embryo by a flattening of the dorsal 



