320 



EMBEYOLOGT. 



changes in the course of the fourth day : the first pair 

 gradually disappears and is at length obliterated, and the se- 

 cond becomes smaller ; but on either side there is a fifth arch 

 formed, which becomes larger on the fifth day, whilst the 

 second now disappears ; so that on this day there are three 

 vascular arches present, all of nearly equal magnitude (fig. 342, 

 A, h l , A 2 , 7i 3 ). The carotid, and by and by the vertebral, arte- 

 ries now make their appearance, arising from the aortal roots, 



and the bulbus aortse 

 undergoes a division in- 

 to two passages. On the 

 fourth day the aorta 

 gives off distinct vessels 

 between the several divi- 

 sions of the vertebrse ; it 

 then divides and fur- 

 nishes two principal 

 branches, which go off 

 in transverse directions 

 (figs. 348 c, 347 i, i, 339 

 m, m, 346,ff), and split- 

 ting intobranchlets, form 

 an extremely beautiful 

 network upon the out- 

 spread germinal mem- 

 brane ; the aorta after- 

 Fig. 347. Embryo of the yolk depicted wards proceeds, first di- 

 in fig. 348, seen from the abdominal as- yided and then single 

 pect, magnified, a, Vagina s. involucrum ^ ^ yertebral & CQ 1 



capitis : b, vagina s. involucrum caude (a ? 



and b, folds of the germinal membrane lumn > g lves off a niesen- 

 enveloping the head and tail) ; c, c, ante- teric artery (figs. 338, 

 rior passage of the involucrum capitis into 343, B, d 5), and finally 

 the lateral involucra ; d, vault of the mass splits into two branches 

 appertaining to the corpora quadrigemina ; 

 e, anterior cerebral mass or lobe; /, heart; 

 y, termination of the venous trunks in the 

 future atrium cordis ; h, aorta ; 1, 2, 3, 4, 

 the four branchial arteries ; z, i, arteries 

 of the blastoderma ; &, A translucent 



that ramify upon the 

 allautois (figs. 341, 345, 

 n) . Almost simultane- 

 ously with the formation 

 of the arteries an accom- 



crests of the dorsal laminae, rendered f 



somewhat \vavy by the water in which 111 \, 



the embryo is immersed ; I, I, vertebral 1S developed ; the veins 



of the germinal mem- 



ammse. 



