202 



EMBRYOLOGY 



the vitelline membrane, has soon after the beginning of incubation 

 become very fluid and its albumen is like the contents of the yolk- 

 sac assimilated into the tissues of the growing embryo. Already 

 a few days before hatching it is used up completely, so' that by 

 this time the embryonic sac and its enclosing membranes fill up 

 the Avhole egg. 



The embryo, as explained above, is formed by a folding-off of 

 the portion of the blastoderm from the yolk-sac. The tubular sac 

 of the embryo, while everywhere acquiring thicker walls, undergoes 

 many modifications through local thickening, budding, and folding, 

 and is gradually moulded into the proper shape of the body of 

 the chick. 



First there appears, on the upper side, a longitudinal canal, the 

 neural tube, the walls of Avhich become transformed into the brain 

 and spinal cord. Below and parallel Avith this tube appears an 

 axis rejDresented by the vertebrae. Underneath this, again, is 

 another tube, closed in above by the axis, and on the sides and 

 below by the body-walls. Enclosed in this second tube, and 

 suspended from the axis, is a third, tube, consisting of the 

 alimentary canal with its diverticular appendages, the liver, 

 pancreas, lungs, etc. The cavity of the outer tube is the body- or 

 pleuro - peritoneal cavity ; it also contains the heart and other 

 parts of the vascular system, together with the genital glands and 



the kidneys, which are all folded or budded- 

 ofF portions of the inner walls of the body- 

 cavity. 



Thus a transverse section of a chick, or in 

 fact of any vertebrate animal, always shews 

 the same fundamental structure ; above a 

 single tube, below a double tube, the latter 

 consisting of one tube enclosed within another, 

 the inner being the alimentary canal, the 

 outer the general cavity of the body. Into 

 such a triple tube the simple tubular embry- 

 onic sac of the chick is converted by a series 

 of changes of a remarkable character. 



The upper or neural tube begins at a 

 very early period by the raising up of the 

 epiblast of the blastoderm into two ridges, 



^R 



Diagrammatic, Transverse 

 Section of the Body of any 

 Vertebrate. 



Ao, Aorta ; c, Peritoneal 

 cavity; g, Gut -cavity; G'j. 

 Genital glands ; A', Kidneys ; 



if, Spinal marrow contained *, ., i,, ii -i:-!. 



in the vertebral column, the Avhlch TUU parallel tO the long aXlS 01 thC 



vertebra and ribs being future embryo and euclose a shallow longi- 

 ^''^'^'''''' tudinal groove. These medullanj folds 



eventually meet and coalesce dorsally in the middle line, thus 

 converting the groove into a canal Avhich becomes closed at either 

 end. The cavity of the tube becomes the cerebro-spinal canal, its 

 Avails are transformed into the spinal cord and through thickenings 



