232 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



it is only when nephrocoeles begin to appear (in the regions of the 



sc 



mi/. 



pn. 



ent. 



Fig. 129. — Development of the pronephros in Lepidosiren as shown in transverse sections. 



A, stage 21; B, stage 21; C, stage 24 + . a.n.d, archinephric duct; end, endoderm ; ent, enteric 

 cavity ; gl, glomerulus ; l.m, lateral mesoderm ; myj myotome ; N, notochord ; nc, nephrocoele ; pw 

 proDephric tubule ; s.c, spinal cord ; sel, sclerotome. 



fifth and sixth segments) that the segmented nature of the rudiment 



