566 



EMBRYOLOGY. 



early period a special investment, the pericardium. In connection 

 with the latter the diaphragm is formed as a partition between the 

 thoracic and abdominal cavities. This is consequently the most 

 suitable place at which to acquaint ourselves better with these 

 important processes, a part of which are not easily understood. The 

 most of the discoveries in this field we owe to the investigations of 

 CADIAT, His, BALFOUR, USKOW, and others. 



(b) The Development of the Pericardial Sac and the Diaphragm. 

 The Differentiation of the Primary Body-cavity into Pericardial, 

 Thoracic, and Abdominal Cavities. 



Originally the body-cavity is widely extended in the body of the 

 embryo, for it can be traced in the lower Vertebrates into tho fun- 

 dament of the head, where it 

 furnishes the cavities of the 

 visceral arches. After the 

 latter have become closed, 

 during which muscles arise 

 from the cells composing their 

 walls, the body-cavity extends 

 forward as far as the last 

 visceral arch and constitutes 

 a large space (fig. 313), in 

 which the heart is developed 

 within the ventral mesentery 

 (mesocardium anterius and 

 posterius). REMAK and KOL- 

 LIKER named this space throat- 

 cavity ; His introduced the 

 name parietal cavity. But it 

 will be most appropriate if 

 one designates it, after the 

 permanent organs which are 

 derived from it, as the peri- 

 cardio - thoracic cavity. The 

 more the cardiac tube is 

 thrown into curves, the more extensive this cavity becomes, and it 

 soon acquires in the embryo a comparatively enoi-mous size. By 

 this its front wall is protruded ventrally like a hernia between the 

 head and the navel of the embryo (figs. 314, 157). 



313. Human embryo (Lg of H:s) 2-15 mm. 

 long, neck measurement. Reconstruction 

 figure, after His(" Menschliche Embryonen "). 

 Magnified 40 diameters. 



Mb, Oral sinus ; Ab, aortic bulb ; Vm, middle 

 part of the ventricle ; Vc, vena cava superior 

 or ductus Cuvieri ; Sr, sinus reunions ; Vu, 

 vena umbilicalis ; VI, left part of the ven- 

 tricle ; Ho, auricle of the heart ; D, diaphragm ; 

 V.om, vena omphalomesenterica ; Lb, solid 

 fundament of the liver ; Lbcj, hepatic duct. 



