194 OUTLINES OF CHORDATE DEVELOPMENT 



of the thickening a continuous space is formed, from which 

 there extend medially or proximally, toward the lateral plate, 

 three small irregular canals which open into the upper margin 



_ FIG. 70. Sections through frog embryos (R. sylvatica) illustrating the forma- 

 tion of the pronephros. After Field. A. Through the anterior body region of an 

 embryo at the commencement of its elongation. B. Through the anterior end 

 of the pronephric rudiment of an embryo in which the neural folds are just 

 closed together. C. Through the second nephrostome of an embryo of about 

 3.5 mm. c, Ccelom; ca, rudiment of pronephric capusle; cc, communicating 

 canal; ec, ectoderm; en, endoderm; g, gut cavity; mp, medullary plate; my, 

 myotome; my>, second myotome; n, notochord; nc, rudiment of neural crest; 

 si, st, first and second pronephric nephrostomes; sc, spinal cord; sn, subnoto- 

 chordal rod (hypochorda) ; so, somatic layer of mesoderm (in A the reference 

 line points to the rudiment of the pronephros) ; S p, splanchnic layer of mesoderm; 

 t, pronephric tubule; v, vertebral plate of mesoderm. 



of the coelom of the lateral plate opposite the middle of each of 

 the three pronephric somites (Field). From the posterior 

 end of this peripheral space or common trunk, the cavity leads 



