DEVELOPMENT OF RANA 177 



archenteron, largely rilled up by the yolk-cells of the vegetative 

 pole, which are visible inside the rim of the blastopore. The 

 cavity of the blastoccel becomes reduced and obliterated as 

 the cavity of the archenteron increases and gastrulation 

 proceeds ; and the yolk-laden cells of the vegetative pole 

 come to lie on the ventral side of the archenteron. 



As the blastopore approaches the vegetative pole its diameter 

 decreases, until when it reaches it, it is a small spherical hole 

 with yolk-cells showing through as the so-called yolk-plug. 



The processes of gastrulation therefore entail overgrowth 

 or epiboly, and invagination ; but the invagination cannot 

 take place simply as in Amphioxus owing to the large quantity 

 of yolk present, and it is more in the nature of an ingrowth. 

 At all events, the result of gastrulation is the conversion of the 

 single-layered hollow ball (blastula) into a double-layered sac 

 (gastrula) ; the outer layer (ectoderm) is formed of the cells 

 of the animal hemisphere and those which have grown over, 

 the inner layer (future endoderm and mesoderm) is formed of 

 the cells which have grown in, and of the yolk-laden cells of 

 the vegetative hemisphere. The latter form most of the ventral 

 and the former most of the dorsal wall of the archenteron. 

 The heaping up of the heavy yolk-cells at the ventral side 

 causes the gastrula to rotate within its membranes, so that the 

 former egg-axis lies more or less horizontal instead of vertical ; 

 the ventral side now points downwards and the dorsal side 

 upwards. 



Mesoderm and Notochord. — The wall of the archenteron 

 contains the cells which are destined to give rise to the noto- 

 chord and to the mesoderm. A strip of cells running along the 

 middle line of the roof of the archenteron is the rudiment of 

 the notochord, and the mesoderm arises as a splitting off (or 

 delamination) of a layer of cells from the remainder of the wall 

 of the archenteron. This layer of mesoderm now separates 

 the ectoderm from the endoderm in most parts of the embryo. 

 Soon, a split arises in the mesoderm layer itself, dividing it into 

 an inner splanchnic layer and an outer parietal or somatic 

 layer. This split is of course the ccelomic cavity. 



The notochord splits off as a solid rod from the surface of 



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