562 EMBRYOLOGY. 



endocardial ridges that project from the partition, in part by corre- 

 sponding growths of the endocardium at their lateral circumference. 

 The membranous projections are comparable with primitive pocket, 

 valves, such as are also established in the bulbus arteriosus (GEGEN- 

 BAUR) ; they constitute the starting-point for the development of 

 the large atrioventricular valves, but furnish, as GEGENBAUR and 

 BERNAYS have shown, only a part the membranous marginal 

 thickening (mk l ) which subsequently disappears almost completely, 

 whereas the compact main part of the valve arises from that portion 

 of the thickened muscular wall of the ventricle itself that surrounds 

 the atrioventricular opening (fig. 311 B mk). 



As was previously stated, in the case of Man the wall of the 

 ventricle during the first months consists of a close spongy network 



Tig. 311. Diagrammatic representation of the formation of the atriovantricular valves. A, Earlier, 



, later condition. After GEQENBAUR. 

 mk, Membranous valve ; mk', the primitive part of the same ; cht, chordae tendinese ; v, cavity 



of the ventricle ; b, trabecular network of cardiac musculature ; pm, papillary muscles ; 



te, trabeculas carneav 



of muscular trabeculse, which are invested by the endocardium and 

 the interstices of which communicate with the small central cavity 

 (fig. 311 A). Such a spongy condition of the wall of the heart 

 persists permanently in Fishes and Amphibia ; in the higher Verte- 

 brates and Man, on the contrary, metamorphoses occur. Toward 

 its external surface the wall of the heart becomes more compact, in 

 that the muscular trabeculse become thicker and the spaces between 

 them narrower, in some parts even disappearing entirely (fig. 311 B 

 tc). The reverse of this process takes place toward the inside. In 

 the vicinity of the atrioventricular opening the trabeculae become 

 thinner and the interstices larger. In this way a part of the thick 

 wall of the ventricle, which looks toward the atrium and encloses the 

 opening, is undermined, as it were, by the blood-current. In this 

 part the muscle-fibres afterwards become entirely rudimentary ; 



