OF VERTEBRATA. 309 



which no new organs are intro- 

 duced whose origin cannot be 

 easily understood without going 

 through all the intermediate stages. 

 In regard to the body proper, two 

 things will be noticed at once: 

 the one is that it has fallen on its 

 side, and the other that it is en- 

 tirely enclosed by the amniotic sac 

 (fig. 209, am*, am 5 ). Another F;O . 



prominent general feature lies in the large area ol blood-vessels 

 (ef, cv, cv l , af l ). These vessels are but an extension of those 

 whose initiatory formation was pointed out in the last stage. 

 The principal chamber of this system, the heart, (A,) has become 

 transformed into a one-sided, nearly double cavity, by folding 

 upon itself and narrowing the space between its two halves. 

 The blood-vessels are hollowed in the subsidiary layer in two 

 sets, which are respectively named the efferent and afferent ves- 

 sels, i. e., those which carry the blood away from the heart and 

 those which bring it to it again. In the former phase these ves- 

 sels were so undetermined in their course, and so intermingled, 

 that the blood, as I stated, merely surged backward and forward 

 in the same channels ; but since that period they have extended 

 more widely and far beyond the body, and are separated into 

 two groups, which are arranged in this wise: taking the heart 

 (h) as a starting-point, the efferent vessels commence with a 

 single current (vn) which passes from the heart backward and 

 along the mid-line of the lower face of the body to its end. 

 From this single one the vessels of the great net-work (vascular 

 area) arise right and left, and pass, with a few forkings (ef) and 

 inosculations, to a considerable distance beyond the body, where 

 they unite in an irregular circular channel, the vena terminalis, 

 (cv, cv 1 ,) which carries the blood forward beyond the head and 



ef, the efferent vessels of the vascular area ; cv, cv*, the circular vessel (vena 

 terminalis) of the vascular area ; af, a/" 1 , the afferent vessel ; am 4 , a/ 5 , the 

 amniotic sac. Original. 



