The cavity of the intestinal umbilicus connects with the 

 intestine, and the gap of the skin umbilicus connects with 

 the cavity of the embryonic body. Baer explained the origin 

 of the abdominal cavity of the embryo with unusual perspi- 

 cacity, stating that "it is nothing other than the union of 

 both the gaps which were formed in the abdominal plates in 

 the third day" (II 6e, p. 112 (67)). 



Directly after the splitting of the abdominal plates, 

 two abdominal cavities form as narrow slits (Fig. 27, 5). 

 Then these slits become wide and between them a communicatiion 

 develops in the anterior part of the embryo near the heart. 

 The separation into layers spreads from the abdominal plates 

 through the umbilicus to the fold surrounding the body of the 

 embryo. Therefore, the abdominal cavity at first was, to a 

 considerable degree, outside the embryo and contained only 

 the heart; later it included the intestine also. 



On the fourth day, the embryo already has distinct 

 rudiments of the extremities and tail. Its neck curves so 

 strongly that the most anterior part of the head represents 

 the transition of the spinal cord into the medulla oblongata. 

 The heart and even the liver still are found in the neck 

 region. 



The digestive tube remains almost completely straight. 

 In its anterior part, the throat cavity is defined, after 

 which comes a narrowed short part, the esophagus, transferring 

 into an elongated widening, the stomach. The duodenum ends 

 at the "anterior passage." In the posterior part, the large 

 intestine still cannot be distinguished from the small one, 

 which ends anteriorly at the "posterior passage"; the 

 posterior-entrance opening in this period has not yet appeared 



The lungs still connect with the digestive canal by 

 means of a short respiratory tube. The liver plates 

 increase in size, then envelope the portal vein. Inside the 

 liver, the liver ducts branch and the veins undergo ramifica- 

 tion. The urinary sac grows quickly in the second half of 

 the fourth day and enters between the serous and vascular 

 layers of the caudal and then of the lateral folds. Now it 

 is transparent, and in its vascular layer the arterial network 

 is distinctly seen. Inside each Wolff's body passes a 

 longitudinal vessel with branchings. 



325 



