THE EARLY DEVELOPMENT OF THE FROG 125 



continuous with the lateral plate but shortly after they are 

 marke4 out, the two regions become separate and the lateral 

 plate itself is never segmented but remains uninterrupted. 

 The coelomic spaces of the somites, the myoccels, then disappear 

 without leaving any trace. These processes are progressive 

 posteriorly, the formation of the coelom, somites, etc., con- 

 tinuing as the mesoderm forms from the primitive streak. 



In the embryo of this period indications of two important 

 structures are present in connection with the mesoderm but 

 not as readily recognizable and definite rudiments; these are 

 the prone phros and the heart. The connections between the 

 second, third and fourth somites and the lateral plate remain as 

 small masses of cells, in close relation with the somatic layer 

 of the lateral plate. These are the cells forming the rudiments 

 of the pronephric tubules. The definite formation of these 

 rudiments and that of the pronephric duct must be left until a 

 later stage is described. Just below the pharynx and in 

 front of the liver, the mesoderm cells are arranged in somatic 

 and splanchnic layers, and between these the coelomic space is 

 well marked on either side, while elsewhere in the ventral region 

 it has not appeared (Fig. 37, B). Between the mesoderm and 

 the ventral side of the pharynx a few loose cells are scattered 

 along. This is the region where the heart is soon to appear 

 but this, too, must be described later. 

 (References to the literature will be found at the end of Chapter III.) 



