242 H. D. Senior. 



PERIOD 3. IN WHICH THE HEART-TUBE IS COMPLETED TO FORM 



CONUS^ VENTRICLE AND ATRIUM^ AND ASSUMES THE 



ADULT POSITION. 



Stage of }^2 somites. 



Fig. 12 is from a reconstruction of the ventral surface of the 

 pericardial region of an embryo of 42 somites. The ventral wall 

 of the pericardial coelom is very thin and to some extent moulded 

 upon the heart-tube, so that the outlines of the latter are clearly 

 indicated. A portion of the splanchnic mesoderm forming the ven- 

 tral wall of the pericardial coelom has been removed over an area 

 mainly to the right of the mid-line. In the area referred to a portion 

 of the heart wall and of the somatic mesoderm are seen near the 

 mid-line. Toward the lateral region of this area the somatic meso- 

 derm and some of the mesoderm of the pharyngeal floor are repre- 

 sented as having been removed in order to uncover the ventral aspect 

 of the entodennal pharynx and of the ventral aorta. 



The heart-tube, which has only just been completed, is cone- 

 shaped; the (venous) base of the cone is directed anteriorly and to 

 the left. There is, as yet no external indication of separation into 

 conus, ventricle and atrium, but the wall becomes progressively 

 thinner in passing from the arterial to the venous end. 



The splanchnic mesoderm, not included in the heart-tube, forms the 

 ventral wall of the pericardial ccelom; it becomes continuous with 

 the wall of the hart-tube at the venous end of the latter. In closing, 

 the two sides of the heart-tube do not appear to blend where the 

 splanchnic mesoderm first meets; some further adjustment occurs in 

 order to bring the columnar area of the latter (and the endocardium 

 in contact with this) within the limits of the tube. Completion of 

 the heart-tube is effected by the blending of splanchnic mesoderm 

 of the right and left sides, and continuity of the ventral wall of 

 the pericardial ccelom is maintained by a similar process ; the two 

 processes together entail loss of the ventral mesocardium. 



Comparison of Figs. 11 and 12 shows there is a large amount 

 of endocardium not included in the heart-tube; also that, after 

 closure of the latter, the endocardium tends to move over to the 

 left side. 



