240 H. D. Senior. 



In the formation of the heart-tube anterior to the aortic region, 

 the splanchnic mesoderm of the right side undergoes active movement, 

 while that of the left remains comparatively passive. The splanchnic 

 mesoderm to the right of the heart-tube axis arises abruptly from the 

 somatic layer to form a crest which moves over to the left ; this 

 crest becomes imminent and falls to the left somewhat in the manner 

 of a wave breaking upon the shore (see Fig. IIC). The splanchnic 

 mesoderm to the left of the axis rises slightly to meet the splanchnic 

 mesoderm from the other side as the latter falls; between the two 

 a tube of splanchnic mesoderm is formed of which the ventral wall 

 is derived mainly from the right side, and the dorsal wall mainly 

 from the left. 



Fig. 11 shows the heart-tube in process of formation, as indicated 

 by a reconstruction of the stage of 36 somites; contact of the two 

 sides has occurred at the posterior (arterial) end. The splanchnic 

 mesoderm on each side (right particularly), for some little distance 

 anterior to the contact area, shows evidence of preparation for bend- 

 ing in the manner described above (Fig. IIC). Heart-tube forma- 

 tion is now in rapid progress ; the posterior ( arterial) end now being 

 complete, the venous end will be progressively formed, from behind 

 forward, along an axis deviating to the left. 



The irregular black line in Fig. 11 indicates the outline of the 

 endocardium; a small quantity of the latter has been included in 

 the heart-tube. 



The heart shown in Fig. 11 (such as it is) is contracting rhyth- 

 mically, and has been doing so for some 10 or 15 minutes. The 

 heart which was quiescent at the stage of 35 somites began beating 

 (after very little preliminary oscillation) at a rate of 52 beats per 

 minute, about 15 minutes before the thirty-sixth somite was com- 

 pletely marked off.^^ 



It may be questioned whether the rhythmical contraction of the 



"Water temperature 62° F. (July 11, 1907). Some evidence has been 

 obtained which suggests that in liigher water temperatures the heart begins 

 to beat at an earlier stage of development (as estimated by the number of 

 somites). In order to exclude the possibility of a miscount of somites the data 

 on which this evidence rests require to be controlled by comparison with the 

 results of further observations, preferably made on eggs of another species. 



