EARLY DIVISION OF SPLANCHNOCOELIC COELOM 861 



and the primitive septum transversum which develop in relation to the con- 

 verging veins of the sinus venosus and the ventro-cephalic growth of the liver 

 rudiment. In other words, a ventral partition is established across the primi- 

 tive splanchnocoelic coelom in a plane which separates the caudal end of 

 the heart (i.e., sinus venosus) from the anterior limits of the liver. This 

 primitive transverse partition partially separates the primitive splanchnocoelic 

 coelom into two main divisions: 



( 1 ) a cephalic compartment, the pericardial cavity, around the heart and 



Fig. 362 — Continued 

 coelom into anterior pericardial and posterior peritoneal areas in early human embryo. 

 The precocious development of the caudal wall of the parietal pericardium obliterates 

 the ventral recesses shown in A-1 previous to septum transversum formation and the 

 outgrowth of the liver rudiment. Communication between pericardial and peritoneal 

 coelomic divisions is possible only through the dorsal parietal recesses (dorsal pericardio- 

 peritoneal canals). (B) Schematic diagram representing the initial division by the 

 lateral mesocardia of the primitive coelomic cavity into anterior pericardial and posterior 

 peritoneal divisions in an embryo of Squalus acanthias 10 mm. long. The liver outgrowth 

 has been extended forward slightly for diagrammatic purposes. (C) Initial division, by 

 the lateral mesocardia. of the primitive coelom in the 72 hr. chick embryo. Due to the 

 depressed condition of the anterior end of the body much of the heart appears in the 

 section below the sinus venosus and lateral mesocardia. However, if the embryo were 

 straightened and the atrium, etc., of the heart pushed forward, the structural conditions 

 would appear much the same as in B. The dorsal parietal recesses appear on either 

 side of the esophagus. (D) Semidiagrammatic section through caudal end of sinus 

 venosus of 22 mm. shark embryo. The dorsal closing folds are developing on either side 

 of the esophagus, thus closing the dorsal recesses. The liver rudiment is expanding within 

 the substance of the ventral mesentery caudal to the heart to form the liver-septum 

 transversum complex. The latter structure obliterates the ventral recesses below the 

 lateral mesocardia. (E) Diagrammatic representation of the forward and ventral growth 

 of the developing liver within the substance of the ventral mesentery to form the liver- 

 septum transversum complex. (See fig. 363D.) Observe: ventral parietal recesses are 

 obliterated by the forward growth of this complex of tissues. The arrow denotes the 

 passageway from the pericardial coelom into the peritoneal coelom through the dorsal 

 parietal recesses (dorsal pericardioperitoneal canals). (F) Early stage in development 

 of human heart and septum transversum showing ingrowth of somatopleural mesoderm 

 between the previously formed caudal wall of the parietal pericardial membrane (see 

 A-2) and the entoderm of the anterior intestinal portal. (Redrawn from Davis, 1927, 

 Carnegie Inst. Public. 380, Cont. to Embryology, 107.) (G) Later stage of human 

 heart development. Mesodermal partition (septum transversum) is present as a thickened 

 mass of tissue below the developing sinus venosus and between the caudal wall of the 

 parietal pericardium and the gut entoderm. (Redrawn from Davis, see fig. 362F, for 

 reference.) (H) Lateral dissection of fifth week human embryo to show ingrowth of 

 liver tissue into thickened' septum transversum. (Redrawn from Patten, 1946, Human 

 Embryology, Blakiston, Philadelphia.) Arrow denotes passageway (dorsal parietal recess; 

 pericardioperitoneal canal; pleural canal) between pericardial and peritoneal coelomic 

 cavities. (I-l) Sagittal section through 15 mm. pig embryo showing thickened anterior 

 face of liver. This thickened anterior face of the liver later separates from the liver 

 as the primary septum transversum (peritoneo-pericardial membrane). (1-2) Higher 

 powered drawing to show condition of anterior face of liver shown in fig. 362, I-l. 

 (J) Transverse section through thorax and pulmonary area of the body of a bird to 

 show position of dorsal pulmonary diaphragm. (Redrawn from Goodrich, 1930, Studies 

 on the Structure and Development of Vertebrates, Macmilian Co., Limited, London.) 

 Observe position of liver lobes in relation to the heart. Compare with fig. 294, G-4 & G-5. 



