374 EMBKYOLOGY OF THE LOWER VERTEBRATES CH. 



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 trabeculae. 



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 

 ^ ne en docardiac 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 



F,o.i78.-Two stages in the wh j ch Ue in ,. the s Pf e between endocardium 



development of th.- i..-:.rt and myocardium. As this proliferation goes 



of Acantkias seeu from the on the endocardium is eventually made to 



v r! t t rals i i Qnl AfterHoch " bul ge- in t the lumen as four prominent 



stetter, 1906.) ,. . _ T . .***** 



endocardiac ridges. In Acanthms (Gegen- 



. ; T, im "inuH *!" baur > 1894 )> one of . the four ridges-that which 

 de.' 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 l'ul'_riri;_r nf the endocardium into the cavity the 

 atrioventricular cushion (anterior .UK I 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 t<>-vth< T so as 

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

 development goes on, they underi-n ni'i;inioriliosis into a pmvlv 

 mechanical and automatic valvular apparatus. In the conus each 

 rid-."- hcci \cavated into a number of pnckct- valves ( w semi- 



lunar" valves), the cavities of \\hich .pm in a head\\ard direct ic n. 

 (in-il and Ot ! l.iin these cavities as bein^ ]>roduccd simply hv 



the had re (f t,he blood hut it Ifi M!\I ahlr to regard 



