FROM TWELVE TO THIRTY-SIX SOMITES 191 



mass, which grows out towards the ectoderm in the form of a 

 blind, solid sprout. The distal end of each turns backwards 

 and unites with the one behind so as to form a continuous cord 

 of cells, which is thus united with the intermediate cell-mass in 

 successive somites by the original outgrowths. This cord of 

 cells is the beginning of the Wolffian duct. Behind the sixteenth 

 somite, the latter grows freely backwards just above the inter- 

 mediate cell-mass until it reaches the cloaca with which it unites 

 about the 31 s stage. 



/!T. nCr 



Fig. 112. — A. Transverse section through the twelfth somite of a 16 s em- 

 bryo. 



B. Three sections behind A to show the nephrostome of the same pro- 

 nephric tubule. 



V. c. p., Posterior cardinal vein. c. C, Central canal. Ms'ch., mesen- 

 chyme, n. Cr., Neural crest. N'st. Nephrostome. n. T., Neural tube- 

 pr'n. 1, 2, Distal and proximal divisions of the pronephric tubule. 



The primary pronephric tubules are originally attached to 

 the nephrotome opposite the posterior portion of the somite, 

 about half-way between the somite and the lateral plate (Figs. 

 112 and 113). The part of the nephrotome between the attach- 

 ment of the primary tubule and the lateral plate is continuous 

 with the primary tubule and forms a supplementary part of the 

 complete pronephric tubule; the remainder of the nephrotome 

 then becomes converted into mesenchyme and the connection 

 with the somites is lost (Figs. 112 and 113). Thus each pro- 

 nephric tubule forms a connection between the W'olffian duct 



