168 THE FOURTH DAY. [CHAP. 



His, and is composed of well-developed tubes without pig- 

 ment. The urinary part forms in both sexes a small rudi- 

 ment, consisting of blindly ending tubes with yellow pigment, 

 but is most conspicuous in the hen. 



The Wolffian duct remains as the vas deferens in the 

 male. In the female it becomes atrophied and nearly dis- 

 appears. 



The duct of Muller on the right side (that on the left side 

 with the corresponding ovary generally disappearing) remains 

 in the female as the oviduct. In the male it is almost 

 entirely obliterated on both sides. 



21. We may return to the changes which are taking 

 place in the circulation. 



On the fourth day, the point at which the dorsal aorta 

 divides into the two branches which we may now call the iliac 

 arteries is carried much further back towards the tail. 



A short way beyond the point of bifurcation, each iliac 

 gives off a branch to the newly formed allantois. It is not, 

 however, till the second half of the fourth day, when the 

 allantois grows rapidly, that these allantoic, or as we may now 

 call them umbilical, arteries acquire any importance, if indeed 

 they are present before. With the increase of the allantois 

 they speedily acquire such a size, that the iliac trunks from 

 which they were given off seem to be mere branches of them- 

 selves. 



The omphalo-mesaraic arteries are before the end of the 

 day given off from the undivided aortic trunk as a single but 

 quickly bifurcating vessel, the left of the two branches 

 into which it divides being much larger than the right. 



During the third day, we saw that the arterial arch 

 running in the first visceral fold became obliterated, the 

 obliteration being accompanied by the appearance of a new 

 (fourth) arch running in the fourth visceral fold on either 

 side. 



During the fourth day the second pair of arterial arches 

 also becomes nearly (if not entirely) obliterated; but a new 

 pair of arches is developed in the last (fifth) visceral fold, 

 behind the last visceral cleft ; so that there are still three 

 pairs of arterial arches, which however now run in the 

 third, fourth and fifth visceral folds. The last of these is as 

 yet small, and together with the slight remains of the second 



