The Development of the Heart in Shad. 245 



plates in a situation approximately corresponding to the lower end 

 of Fig. 12. The ccelom anterior to the orifices become pericardial; 

 posterior to them it becomes peritoneal ; the coelom between the two 

 orifices ceases to exist. ^* Since separation of the pericardial from 

 the peritoneal portion of the coelom occurs at a situation in which 

 the lateral plates are still separate, the posterior end of the peri- 

 cardium extends farther backward on each side than in the mid-line. 



At the stage of 6.2 mm. the caudal aorta and caudal vein replace 

 the anterior end of the cord of cells found in the tail at 42 somites, 

 and the blood contains a very few corpuscles. Blood passes from 

 the dorsal aorta through the short caudal aorta and then forward 

 through the caudal vein. The caudal vein meets the cardinals^^ near 

 the anus, and from the point of junction two veins pass ventrad 

 (embracing the gut) to join the subintestinal vein ; through these 

 two veins most (or all) of the blood from the caudal vein enters the 

 subintestinal. 



The subintestinal vein is now involved in the rapidly growing 

 liver ; its extreme anterior end {vena reveliens of liver) is free, and 

 discharges its blood ventral to the peritoneum. Blood is now re- 

 tained in the space ventral to the peritoneum (supra vitelline sinus) 

 by the very agency which formerly prevented its flowing there, i. e., 

 by the lateral attachment of the peritoneum to yolk. At the site 

 of discharge of the jugular veins the blood from these meets that 

 flowing from the supravitelline sinus ; and the blood from these two 

 sources enters the venous end of the heart. There appears to be no 

 special mechanism for retaining the blood in the space between the 

 ventral wall of the pericardial coelom and the yolk, for the pericardial 

 plate is attached peripherally to the ectoderm and not to yolk. 



The condition of tJie circulation in the stage described in the in- 

 troduction differs from that in the stage just described in that the 

 cardinal veins are fully formed. The blood from the, now practi- 



"Conditions at the site of discharge of the jugular veins are much compli- 

 cated by the fact that the mesenchyme of the pectoral fins is arising from the 

 somatic mesoderm in this region. A more thorough study will be undertaken 

 later in connection with the veins themselves. 



"The cardinal veins, at this stage extend only from the anus as far as the 

 anterior end of the liver. I am unable to determine their functions in connec- 

 tion with the general circulation. 



