530 EMBRYOLOGY OF THE LOWE K' V K 1 ; T K li KATES OH, 



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 (N) lying immediately 



s.c. 



N 



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 ; xom, somatopleure ; spl, splanchnopleure ; splc, splanchnococlr. 



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 



end.. 



!'.'> IB. Transverse section through a second-day Fowl embiyo just In-hind the hinder 

 limit of tli<- 1'nregut. 



A, dornal aorta; end, endoderm; my, myotome; N, notoHionl ; 1.0, -pinal i-or.l ; fum. som:itopli'im> ; 

 >/./, splanchnopleure ; xjdr, splani-liiiiicorlf ; ,-, irenwll "I \:i*riil;ir area. 



in concordance with I In- fact that development pneeels fnun tin- 

 head end tailwards. 'I'hus the neural tube opens <>ui l.y tin- slit.-like 

 rliomlioidal sinus; tlie arohinephrio duct disappears; the notochord 

 passes back into the andiffexentiated tissue of the ])rimitivc streak. 



On tin- niht-i liand llieexaniinalK.il !' sect ins 1'ai't her lr\vanl 

 towards tin- 1 ion l.rin^s into \ie\v vai'i.iiis iinportaiit. lurlher 



devel' Such a section U 'hat sho\\n in Ki'j. L': 1 ,!!; illustrates 



