312 THE DEVELOPMENT 



there branching and reuniting, as in the head, passes forward 

 in a single current (vn 2 , vn 3 ) along each side of the rnid-line of 

 the body to the posterior end of the auricle (A 1 ) of the heart. The 

 currents of the other set arise at several points, right and left, along 

 the abdominal aorta, (ao 3 ,) and pass out of the body in numerous 

 channels, (ef, ef l , e/ 2 ,) of varying calibre, into the great net-work 

 of vessels of the vascular area, and thence into the as yet devel- 

 oping, half-formed vessels in the depths of the yolk mass. From 

 these the blood is gathered into return currents, which finally 

 unite in the great afferent vessel, (a/", a/* 1 ,) that passes into the 

 abdominal cavity just behind the heart, and empties its contents 

 into the auricle (A 1 ) at the same point with the termination of - 

 other afferent vessels (vn l , wn?). The approximating sides (fl) 

 of the body have not only narrowed the lateral extent of the 

 general cavity, but have closed over a considerable space just in 

 front of the tail. In consequence of this, the membrane or layer 

 over which the vessels of the vascular area are spread is nar- 

 rowed to a sort of neck (ys]. In view of this we might say that 

 the yolk is contained in a great saccular prolongation of the 

 sides of the abdominal region ; and in consideration that the 

 blood-vessels plunge through it in every direction, and divide it 

 into a sponge-like mass, the name nutrient organ would not be 

 inappropriate for it; at least, it will serve to indicate 'its nature 

 and peculiar function at this period of life. 



There is no conspicuous change in the development of that 

 part of the nervous cord which lies behind the head, but that 

 portion which lies in front of the ear, (e,) and corresponds to the 

 brain, has commenced the division of its cavity into two cham- 

 bers, by slight constrictions. The first division extends from the 

 end (cr) of the head backwards to a point nearly opposite the 

 end (ch) of the chorda dorsalis, and corresponds to the cerebral 

 region of the adult (see p. 230, fig. 134) ; the second division 

 reaches from the termination of the last to the ear, (e,) and an- 

 swers to two parts of the brain ; namely, in front, to the posterior 

 lobes of the cerebrum, and behind, (w,) in the neighborhood 

 of the ear, to the cerebellum, or posterior region of the brain (see 



