THE DIGESTIVE CANAL. 



59 



As soon as the head of the embryo has grown so much as to project as an in- 

 dependent part, we find that the oral plate lies on the under surface of the head, a 

 little in front of the heart (Fig. 26). The pro-amnion, pro. am, arises from the 

 somatopleure enclosing the heart, ht, so that, when the oral plate becomes perforate, 

 the cavity of the entoderm, Ent, will communicate directly with the cavity enclosed 

 by the pro-amnion, or, in other words, with the permanent amniotic cavity. Figure 

 72, o.pl, shows the oral plate in a little later stage, shortly after which the plate 

 ruptures. 



A similar anal plate at the 

 posterior end of the embryo also 

 lies within the amnion (Fig. 21). 

 This figure is taken from a sheep 

 embryo in a very early stage, so 

 that the anal plate appears to lie 

 on the dorsal side. By the curl- 

 ing ventralward or the bending 

 over of the tail end of the young 

 embryo the anal plate is gradually 

 transferred or rolled over on to 

 the ventral side, where it perma- 

 nently remains, For the relation 

 of the anus to the blastopore see 

 page 54. 



pro.anv 



FIG. 26. LONGITUDINAL SECTION OF THE HEAD END OF A 



MOLE EMBRYO, STAGE H. 



EC, Ectoderm. En, Entoderm. Ent, Anterior end of archenteric 

 cavity with the oral plate on the cardiac side, fb, Fore- 

 brain. ht, Heart, m.b, Mid-brain. Mes, Mesoderm. nch, 

 Notochord. pro.am, Pro-amnion. (After W. Heape.) 



The Digestive Canal. 



The digestive canal proper is developed by the growth and modifications of the 

 fore-gut and hind-gut. The division between the two is a point in the ileum corre- 

 sponding to the original connection with the yolk-sac, marked in the fetus by 

 Meckel's diverticulum. 



The fore-gut forms the pharynx (and lungs), the oesophagus, stomach, duode- 

 num, and part of the ileum. It also produces, as appendages to the canal, the 

 liver and pancreas. 



The hind-gut forms most of the ileum and the entire large intestine, together 

 with the caecum and appendix. 



The entoderm persists as the permanent epithelial lining, and produces all the 

 glands of the digestive tract. It remains a thin layer throughout life. The meso- 

 derm forms the greater part of the walls, furnishing the connective tissue, the 

 smooth muscle layers, and the peritoneum, which last consists of the original meso- 

 thelium and a thin layer of chiefly fibrillar connective tissue. 



The general course of the development is shown by figure 27, which represents 

 outlines of the entodermal canal in three human embryos, uniformly magnified 

 twelve diameters. An earlier stage is shown in figure 25. 



