STRUCTURE OF FIFTY-HOUR CHICKS 99 



vesicle. But these primordia once separated from the ectoderm 

 follow divergent lines of differentiation leading to adult condi- 

 tions which are structurally and functionally totally unlike. 

 The origin of these two structures from cell groups similarly 

 folded off from the same germ layer, but which once established 

 undergo each their own characteristic differentiation, exempli- 

 fies a sequence of events so characteristic of developmental 

 processes in general as to call for at least a comment in passing. 



The Posterior Part of the Brain and the Cord Region of the 

 Neural Tube. — Caudal to the diencephalon the brain shows no 

 great change as compared with the last stages considered. The 

 mesencephalon is somewhat enlarged and the constrictions 

 separating it from the diencephalon cephalically and the 

 metencephalon caudally are more sharply marked. The meten- 

 cephalon is more clearly marked off from the myelencephalon 

 and its roof is beginning to show thickening. In the myelen- 

 cephalon the neuromeric constrictions are still evident in 

 the ventral and lateral walls (Figs. 34 and 35). The dorsal wall 

 has become much thinner than the ventral and lateral walls 

 (Fig. 36, ^ and B) and shows no trace of division between the 

 neuromeres. 



In the cord region of the neural tube the lateral walls have 

 become thickened at the expense of the lumen so that the 

 neural canal appears slit-like in sections of embryos of this age 

 (Fig. 36, E) rather than eliptical as it is immediately after the 

 closure of the neural folds. At this stage the closure of the 

 neural tube is completed throughout its entire length. The 

 last regions to close were at the cephalic and caudal ends of the 

 neural groove. In younger stages where they remained open 

 these regions were known as the anterior neuropore and the 

 sinus rhomboidalis, respectively. . 



The Neural Crest. — In the closure of the nairal tube the 

 superficial ectoderm which at first lay on either side of the 

 neural groove, continuous with the neural plate ectoderm, 

 becomes fused in the mid-line and separated from the neural 

 plate to constitute an unbroken ectodermal covering (Cf. Figs. 

 17, B, and 28, B). At the same time the lateral margins of the 

 neural plate become fused to complete the neural tube. There 

 are cells lying originally at 1 he edges of the neural folds which 

 are not involved in the fusion of either the superficial ectoderm 



