328 ORR. [3^01. I. 



ward below the intestine, and unite the apical surfaces of the 

 folds in the median line just ventral to the intestine. A little 

 distance below this another similar in-folding takes place ; 

 and thus arises a tubular lumen enclosed by the splanchno- 

 pleures, between the head-intestine and the hypoblastic wall of 

 the yolk-sack. Fig. i6, A, shows this organ in process of for- 

 mation : HT represents the lumen of the tube, — eventually the 

 lumen of the heart. Owing to the twist of the embryo the 

 growth is very indistinct on one side. When the tube is com- 

 pletely enclosed, it becomes separate from the splanchnopleures 

 above and below it. These, however, remain continuous with 

 each other across the median line, above and below the tube. 

 Later, the growth of the somatopleure and epiblast, encircling 

 the anterior ventral part of the body, separates this tube from 

 that layer of the splanchnopleure which is adjacent to the yolk- 

 sack. The ends of the tube open to the spaces between the 

 hypoblast and mesoblast. As the tube grows in size it folds 

 over on itself and forms a ventral loop, assuming at the same 

 time the appearance and functions of a heart (see Fig. 59, B, 

 and series C). The pericardial cavity is by this time very 

 large, extending forw^ard to the lower lip of the mouth. En- 

 closed courses have also appeared for the circulation of the 

 blood. The blood-vessels, for the greater part of the body, may 

 be traced out in series C. At first, all the blood from the heart 

 passes through the mandibular arteries which divide where the 

 ventral trunk leaves the pericardial cavity. At this time none 

 of the posterior arterial arches have appeared. The mandibular 

 arteries empty into two dorsal arteries. Anterior to the conflu- 

 ence of the mandibular arteries with the dorsal arteries the lat- 

 ter are continued (with decreased lumen) forward into the head 

 as the carotids. In the region of the Wolffian organs the two 

 dorsal arteries become a single median trunk, but, again, in 

 the tail appear as two distinct arteries. This paired condition 

 of the dorsal arteries is interesting, from the fact that previous 

 to the appearance of any walls to the blood-vessels, the blood- 

 corpuscles are found throughout the length of the body in the 

 paired spaces on either side of the notochord, these spaces 

 being completely separated by the medulla, notochord, and 

 hypoblast. 



The cardinal veins are continued in unbroken course forward 



