320 



EMBRYOLOGY. 



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 t h 2 , A 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 vertebrae ; 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 into bran chlets, form 

 an extremely beautiful 

 network upon the out- 

 spread germinal mem- 

 brane ; the aorta after- 

 wards proceeds, first di- 

 vided and then single, 

 along the vertebral co- 

 lumn, gives off a mesen- 

 teric artery (figs. 338, 

 343, B, d 5), and finally 

 splits into two branches 

 that ramify upon the 

 allantois (figs. 341, 345, 

 n). Almost simultane- 

 ously with the formation 

 of the arteries an accom- 



Fig. 347. Embryo of the yolk depicted 

 in fig. 348, seen from the abdominal as- 

 pect, magnified, a, Vagina s. involucrum 

 capitis : b, vagina s. involucrum caude (a 

 and 6, folds of the germinal membrane 

 enveloping the head and tail) ; c, c, ante- 

 rior passage of the involucrum capitis into 

 the lateral involucra ; d, vault of the mass 

 appertaining to the corpora quadrigemina ; 

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

 g, termination of the venous trunks in the 

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

 the four branchial arteries ; i, i, arteries 

 of the blastoderma ; k, k, translucent 

 crests of the dorsal laminae, rendered 



somewhat wavy by the water in which P an y in S Astern of veins 



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



aminEe. of the germinal mem- 



