530 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



the mesoderm segment. Immediately above the nephrotome, between 

 it and the ectoderm, is seen the rudiment of the archinephric duct — 

 a rod of cells which is gradually extending tailwards. 



In the centre of the section is the neural tube (s.c) with its thick 

 walls and the solid notochordal rudiment (iV) lying immediately 



Fig. 234a. — Transverse section through the middle of a second-day Fowl embryo 



(15 segments). 



A, paired dorsal aorta; a.n.d, archinephric duct; ect, ectoderm; end, endoderm; my, myotome; 

 N, notochord ; s.c, spinal cord ; som, somatopleure ; spl, splanchnopleure ; splc, splanchnocoele. 



below it. The blood-vessel (A) on each side between nephrotome 

 and endoderm is the dorsal aorta which is at this stage double. 



Working back towards the tail end of the embryo it is seen that 

 subsequent sections show less and less advanced stages of development 



I mi h n 1 1 



Fig. 234b. — Transverse section through a second-day Fowl embryo just behind the hinder 



limit of the foregut. 



A, dorsal aorta ; end, endoderm ; my, myotome ; N, notochord ; s.c, spinal cord ; som, somatopleure ; 

 spl, splanchnopleure ; splc, splanchnocoele ; i\ vessels of vascular area. 



in concordance with the fact that development proceeds from the 

 head end tailwards. Thus the neural tube opens out by the slit-like 

 rhomboidal sinus ; the archinephric duct disappears ; the notochord 

 passes back into the undifferentiated tissue of the primitive streak. 



On the other hand the examination of sections farther forward 

 towards the head region brings into view various important further 

 developments. Such a section as that shown in Fig. 234b illustrates 



