374 EMBEYOLOGY OP THE LOWEK VEETEBEATES oh. 



towards the condition of the fully developed heart is marked mainly 

 by the increase in relative size of the atrium and ventricle. Whereas 

 however the increase in the size of the atrium takes the form mainly 

 of a mere process of dilatation, that of the ventricle is accompanied 

 by a much more marked thickening of its wall. This is brought 

 about by the inner surface of the myocardium forming numerous 

 projections into the lumen which, becoming more and more pro- 

 nounced and interlacing and fusing with one another, form eventu- 

 ally a spongework and encroach considerably on the ventricular 

 cavity round its periphery. The endocardium fits closely over the 

 surface of each of these myocardiac trabecular 



The physiological meaning of the formation of the trabeculae 

 during the evolution of the ventricle probably lies in the fact that a 

 bundle of muscle which has for its function 

 the pulling together of the ventricular wall 

 can carry out this function more efficiently if 

 it runs straight between its two ends, in other 

 words if it is in the position of a chord to 

 the curve of the ventricular wall rather than 

 simply a portion of that curve. 



Attention must now be directed to a very 

 characteristic and important proliferation of 

 the endocardiac cells which makes itself 

 apparent in particular regions. In the conus 

 such proliferation takes place along the course 

 of four longitudinal lines, giving rise' to cells 



Fio.i78.-Two stages in the wh , ich lie in the s P ace between endocardium 

 development of the heart and myocardium. As this proliferation goes 

 of Aamthias seen from the on the endocardium is eventually made to 



ru" 6 .r erH ° 0h " bul § e int0 the lumen as four prominent 

 endocardiac ridges. In Acanthias (Gegen- 



at, atrium; c, conus arterio- t^„„„ tor\A\ — c j.i r ■ i , -i , ?■ t 



sus; s .v, sinus venosns ; v, paur, 1894), one of the four ridges— that which 

 ventricle. is ventral in position — is reduced in size. 



In the auricular canal similar endocardiac 

 proliferations take place, one upon the headward and one upon 

 the tailward wall respectively of the canal. Here also each causes 

 a prominent bulging of the endocardium into the cavity— the 

 atrioventricular cushion (anterior and posterior). 



Both the ridges of the conus and the atrioventricular cushions 

 constitute a valvular apparatus in that, by the contraction of the 

 myocardium lying outside them, they can be jammed together so as 

 to occlude the lumen into which they project. In both cases, as 

 development goes on, they undergo metamorphosis into a purely 

 mechanical and automatic valvular apparatus. In the conus each 

 ridge becomes excavated into a number of pocket valves ("semi- 

 lunar " valves), the cavities of which open in a headward direction. 

 Greil and others explain these cavities as being produced simply by 

 the backward pressure of the blood but it is advisable to regard 



