THE LATER DEVELOPMENT OF THE FROG 171 



m 



In the region of the head and pharynx the mesoderm is 

 in the form of scattered groups of cells, mesenchymal in charac- 

 ter, filling the irregular spaces among the organs of these 

 regions, brain, sense 

 organs, ganglia, gill- 

 pouches, etc. Some of 

 the details of the later 

 history of this mesoderm 

 have already been men- 

 tioned in connection 

 with the visceral 

 pouches, and others will 

 be considered with the 

 history of the vascular 

 and skeletal systems. 

 We may mention here, 

 however, the essential 

 facts regarding the 

 development of the 

 somites and lateral 

 plate. 



The formation and 

 differentiation of the 

 somites and lateral plate 

 occur progressively in 

 the posterior direction, 

 so that in a young larva 

 all of the process may 



* 4 * 



be read in a Series Of 



A , 



transverse Sections. 



The Cavity Of the SO- 



. ,. 

 mite, the myOCOel, lies 



toward its surface; the 

 outer wall, only one cell thick, forms the cutis plate or derma- 

 tome, lying just beneath the surface ectoderm (Fig. 60). 

 The inner wall of the somite is much thickened as the 

 myotome or muscle plate; through the continued thickening 



_ __ , 



FIG. 60. Transverse section through the 

 sixth mesodermal somite of a 5 mm. larva of R. 

 temporaria, illustrating the arrangement of the 

 meso derm. From Maurer (Hertwig's Hand- 

 buch, etc.). c, Cutis plate; Ch, notochord; D, 

 gut wall; m, myotome (muscle plate) ; we, nerve 

 cord; p, lateral plate ;, ventral process of my o- 



torae and cutis plate ' 



