776 



DEVELOrMEXT OF THE ALIMEXTxiRY CANAL. 



1 



the narrow opening by which the gradually lengthening ductus vitello- 

 intestinalis leads into the umbilical vesicle. The middle part of the 

 intestinal canal has, when first produced, more or less the form of a 

 straight tube lying close to the vertebral column ; but as it increases 



Fi?. 577. 



Fig. 577. — Human Embryo of TniRxr-FiVE 

 DAYS SEEN FROM BEFORE (from Kcilliker after 

 Coste). 



3, left external nasal process ; 4, superior 

 maxillary process ; 5, lower maxillary jirocess ; 

 z, tongue ; b, aortic bulb ; b', first perrmmeut 

 aortic arch, which becomes ascending aorta ; 

 b" , second aortic arch ; b"' , third aortic arch or 

 ductus Botalli ; ?/, the developing pulmonary 

 arteries ; c, the superior cava and right azygos 

 vein ; c', the common venous sinus of the heart ; 

 c", the common stem of the left vena cava and 

 left azygos ; o', left auricle of the heart ; v, right 

 v', left ventricle ; a e, lungs ; c, stomach ; j, 

 left omphalo-mesenteric vein ; s, continuation 

 of the same behind the pylorus, which becomes 

 afterwards the vena portaj ; x, vitello-intestinal 

 duct ; a, right omphalo-mesenteric artei-y ; m, 

 Wolffian body ; ?', rectum ; n, umbilical artery ; 

 Vi umbilical vein ; 8, tail ; 9, anterior, U', 

 jjosterior limb. The liver has been removed. 



in length, it is thrown into the shape 

 of a loop bent downwards to the 

 umbilicus, — a change which is ac- 

 companied by the formation of the 

 mesentery. The latter structure is 

 undoubtedly entirely due to splanchno- 

 pleure mesoblastic elements, which, 

 extending themselves between the pro- 

 to-vertebral masses and the elongating 

 gut, become developed into the vascu- 

 lar and other parts of the mesentery, 

 as was long ago shown by Yon Baer. 

 But the mesoblast, also, by its splan- 

 chnopleure division, furnishes the con- 

 tractile fibrous, vascular, and con- 

 nective tissue elements of the intestinal walls. The extent to which 

 the glandular elements of the alimentary canal are supplied by the 

 hypoblast, to which their origin was entirely attributed by Remak, or 

 furnished rather by mesoblast from the proto vertebral mass, as held by 

 Schenk, is not yet determined. 



As develo[)ment proceeds in the forepart of the alimentary canal, a 

 change in its form manifests itself, by which one part, becoming dilated, 

 forms the commencement of the stomach, while the others remain of 

 smaller diameter as gullet and duodenum ; and in connection with dif- 

 ferent parts of these 'the rudiments begin to appear of the lungs, liver, 

 and pancreas. 



When the tubular parts of the gut have attained to some length, a 

 change of position gradually accompanies their further development. 

 While the oesophageal part remains comparatively straight, the dilated 



