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 31s stage. 
LG 
a7 
her 
Pats 
en 
tis = a 
_ Soil Ys LF ee 
CRT Fn Pre p “2 
Fig. 112. — A. Transverse section through the twelfth somite of a 16s em- 
bryo. 
B. Three sections behind A to show the nephrostome of the same pro- 
nephric tubule. 
V. ce. 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 (Iigs. 
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- 
nephrie tubule forms a connection between the Wolffian duct 
