310 



THE DEVELOPMENT 



empties it into a large single vessel, the afferent vessel, (af, of 1 ,) 

 by which it is transported directly to the posterior chamber 

 (auricle) of the heart. 



By the increased incurvation of the head (hd) and tail (/), and 

 the infolding of the sides of the body, the space which lies be- 

 t-ween these parts is pretty nearly isolated and shaped into a body- 

 cavity. At its anterior region this is entirely closed over, from 

 the head to the point, just behind the heart, where the afferent 

 vessel (a/", of 1 ) enters. The preliminary vertebraB are devel- 

 oped along nearly the whole length of the body in a backward 

 direction, and, as a natural consequence, the tubular nervous 

 cord, (,) or spinal marrow, has likewise closed over to an equal 

 extent. In front, the vertebrae have extended only a short dis- 

 tance, in fact, as far as they will ever appear as distinct bodies. 

 At this period the eyes and ears are so far advanced in develop- 

 ment as to be clearly recognizable in their respective positions. 



But one stage further we find the relations of the organs and 

 their state of development very much in the condition of those of 

 some of the more lowly organized fishes, and, in certain respects, 

 like those of the Lancelet, (p. 226, fig. 133,) which I described 

 in a previous lecture. This resemblance is especially noticeable 



vn 3 oo 3 v 1 fl ef cA 2 af vn^ vn\ A 1 



af 1 cr 

 Fig. 210. 



ey ao 1 en 



Fig. 210. The same as %s. 204 to 209, but more highly developed. 6 diam. 

 A profile view of the body proper and a part of the vascular area. The anmi- 

 otic sac has been removed, ey, the eye ; e, the ear ; cr, the anterior end of the 



