860 



THE ALIMENTARY CANAL. 

 Fig. 602. 



Fig. 602, A. DIAGRAMMATIC SECTION SHOWING THE RELATION IN A MAMMAL AND IN 

 MAN BETWEEN THE PRIMITIVE ALIMENTARY CANAL AND THE MEMBRANES OF THE 

 OVUM. 



The stage represented in this diagram corresponds to that of the fifteenth or seven- 

 teenth day in the human embryo, previous to the expansion of the allautois : c, the 

 villous chorion ; a, the amnion ; a', the place of convergence of the amnion and reflection 

 of the false amnion a" a", or outer or corneous layer ; e, the head and trunk of the 

 embryo, comprising the primitive vertebrse and cerebro-spinal axis ; i, i, the simple 

 alimentary canal in its upper and lower portions ; v, the yolk-sac or umbilical vesicle ; 

 v i, the vitello-intestinal opening ; w, the allantois connected by a pedicle with the anal 

 portion of the alimentary canal. 



Fig. 602, B. TRANSVERSE SECTION OP THE BODY OP AN EMBRYO, WITH A PART OP THE 

 ADJACENT MEMBRANES, SHOWING THE RELATION OP THE ALIMENTARY CAVITY TO THK 

 LAYERS OP THE GERMINAL MEMBRANE (from Remak and Kolliker). ^ 



1, chorda dorsalis ; 2, 3, spinal marrow ; 4, cuticular layer, and within it the pi-im- 

 ordial vertebral segments ; 5, the ventral or visceral plates, consisting of the cuticular 

 layer and the outer lamina of the middle germinal layer, passing at 4 x 5 from the 

 umbilicus into the amnion ; 5', within the embryo, is placed in the peritoneal cavity, 

 below one of the Wolflian bodies and close to the musculo-intestinal lamina ; 6, cavity of 

 the intestine lined by the epithelial or epithelio-glandular layer, which, along with the 

 musculo-intestinal, is continued by the ductus vitello-intestinalis into the yelk-sac, 5' 6. 



mesentery. Soon, however, the intestine, growing in length, advances from the spine, 

 and forms a simple loop in the middle of the body, with a straight portion at its 

 upper and lower end, and at the same time becomes slightly dilated in the part destined 

 to form the stomach. The middle of the loop is connected with the umbilical vesicle 

 by the pedicle, and also by the omphalo-mesenteric vessels. The upper extremity of 

 the primitive alimentary tube reaches to the base of the skull and forms the oesophagus 

 and pharynx ; but the mouth is developed by depression of the outer surface of the 

 embryo, above the first branchial arch, and together with the tongue is at first sepa- 

 rated from the throat by a partition, which soon gives way. In like manner, the anal 

 orifice does not exist at first, but is formed by invagination of the outer surface, and 

 the opening of a communication between it and the intestine. 



