THE DEVELOPMENT OF LEl'IDO.SlU KN TAUxiDOXA. 



21 



yolk in the cells of this ridge is usually in a state of com- 

 paratively fine subdivision, though much coarser than that 

 of the epiblast. 



A set of division plaiies now become so arranged as to mai-k 

 off tlie notochordal part of the ridge from the comparatively 

 thin basal layer next the cavity of the archenteron (fig. 17, 

 e. r.). The cells of this latter frequently, though by no 

 means always, retain their yolk in a coarse-grained condition. 



Am £t:6 



Text-i'IG. 1. — Transverse section tliroui,^li dorsal region of an enil)ryoof Stage 

 23. a.d. Arcliinephric duct. enl. Enteron. h.m. iMeser.cliyme cells being 

 directly split off from hypoblast, n. Notocliord. scl. Sclerotome out- 

 growth from mesoblast. 



They are part of the definitive hypoblast, and form the roof 

 of the enteron. The enteric roof is thus differentiated in 

 situ from the cells of the archenteric roof, Avithout any trace 

 of ingrowth from the sides such as has been described by 

 Lwoff, Brauerj and others. 



The notochordal rudiment thus laid doAvn retains for some 

 time its compai-atively undifferentiated condition (figs. 20 

 and 21), showing no obvious change beyond assuming a 

 rounder, more definite outline as it separates off the hypo- 

 blast. About Stage 2o the separation is completed, and the 



