l82 



EMBRYOLOGICAL TYPES 



Median to the myotomes, cells are proliferated by the 

 somites to form clouds of mesenchyme surrounding the nerve- 

 tube and notochord. These cells are the sclerotomes (likewise 

 segmental) from which later on the vertebrae are developed. 



Another instance of segmentation will be seen in connexion 



Fig. 8 i . — Transverse section through a 

 young tadpole larva of Rana showing 

 the origin of the kidneys. 



Fig. 82. — Transverse section through a 

 tadpole larva of Rana older than the 

 previous, showing the formation of 

 the kidneys and the lungs. 



The section'passes through the transverse septum across which the duc- 

 tus Cuvieri lead from the cardinal veins to the heart, c, ccelom dorsal to 

 the transverse septum ; cv, cardinal vein ; dC, ductus Cuvieri ; g, gut ; gl, 

 glottis ; icm, intermediate cell-mass or nephrotome ; /, lung ; Ida, lateral 

 dorsal aorta ; Ir, liver ; m, mesoderm ; my, myotome ; n, notochord ; nc, 

 nerve-cord ; tier, neural crest ; nl, nephrocoel ; pt, pronephric tubules ; 

 sp, splanchnoccel ; y, yolk-cells. 



with blood-vessels, which run transversely in the septa between 

 adjacent segments. Although in the adult animal much of 

 this segmentation is obscured and modified, it is important 

 to note that in development, metameric segmentation is as 

 well marked as in Amphioxus, except for the splanchnoccel. 



