mi: OBOAHI 01 mi: Q 



II. Differentiation of the Alimentary Tube into Separate Regions 

 and Formation of the Mesenteries. 



At first the alimentary tube is broadly in OOntftd (tig. 11G) with 

 the dorsal wall of tin- trunk: it is unitl to tin- clturda (c/t), the 

 neural tube, and the primitive segment! by means of a broad n., 

 embryonic connective tissue, in which tin- fundaments of two large 



hlood- \vssels, the primitive aortic (ao) t are -ncl- d. Tin- right 

 lift portions of the body-cavity an- therefore still >eparatd from 

 each other on the dorsal side by a considerable distance. Tin- older 

 tin- embryo is, the less this distance becomes, until then- iv>ult> a 

 mesentery, a structure which is establish* d along the whole length of 

 the intestinal tul>e. with exception of the anterior portion, in the 

 following manner (compare, Plate I., figs. 8 and D with fig. 10). The 

 alimentary tube recedes from the chorda; at the same time the broad 

 tract of connective tissue previously mentioned becomes narrower 

 from right to left, but elongated dor-.. \ -nt rally (fig. 10, Plate I.) ; 

 the two aortae embraced in it move nearer and nearer together ai.d 

 finally fuse into a single trunk, which lies in the median plane between 

 chorda and intestine. After the further advance of this proce^ the 

 alimentary tube and chorda remain united by means of only a thin 

 hand, which stretches from the front to the hind end of the embryo. 

 This procei d> from the connective tissue enveloping the cl.< 

 encloses along its line of origin the aorta, and is composed of three 

 layers: a connective-tusue lamella, in which blood-vessels run to 

 the inti-tine. and two epithelial coverings, which are derived from 

 the middle germ-layer and are now composed of greatly flattened 



cells. 



The differentiation of tJte alln /,////// Me into .-fit-ate non-> 

 regions lying one behind the other begins with the development of the 

 stomach. This first becomes distinguishable, at >ome distance be- 

 hind the respiratory tract, as a small >pindle shaped enlargement, the 

 long axis of which corresponds with that of the body (iig>. It'.'J and 

 163 M'/). Such a condition i> attained by the human euibrx o of the 

 fourth week. Five succe>-i\e regions may now be di>th:gni>h. d in 

 the whole embryonic alimentary tube: the oral cavity, the tl 

 cavity with itr- Visceral clefts, which i- narrow, d into - of a 



funnel when- it merges into [the third region,] the gull. t. Ti 

 followed by the spindle-shaped t nlarg. ment , ; !. .-toinach. and tin- 

 latter by the remaining portion of the alimentary tube, which .--till is 

 more or le.-s bnadly connected (Ds) with the yolk->ac. Excepting 



