106 THE SECOND DAY. [cHAP. 



not yet completed; and the fluid which is at first driven 

 by the heart contains, according to most observers, very 

 few corpuscles. 



At the close of the second day the single pair of 

 aortic arches into which the bulbus arteriosus divides 

 is found to be accompanied by a second pair, formed 

 in the same way as the first, and occupying a position a 

 little behind it. Sometimes even a third pair is added. 

 Of these aortic arches we shall have to speak more fully 

 later on. 



Wolffian duct. During the latter half of the second 

 day the Wolffian duct to which we have already alluded 

 becomes fully established, while the first traces of thu 

 embryonic excretory organs or kidneys, known as the 

 Wolffian bodies, make their appearance. The develop- 

 ment of the latter will be dealt with in the history of 

 the third day, but the history of the duct itself may 

 conveniently be completed here. 



The first trace of it is visible in an embryo Chick 

 with eight somites, as a ridge projecting from the inter- 

 mediate cell mass towards the epiblast in the region of 

 the seventh somite. In the course of further develop- 

 ment it continues to constitute such a ridge as far as 

 the eleventh somite (Fig. 34 Wd), but from this point it 

 grows backwards by the division of its cells, as a free 

 column in the space between the epiblast and mesoblast. 

 In an embryo with fourteen somites of about the 

 stage represented in fig. 28 a small lumen has appeared 

 in its middle part, and in front it is connected with 

 rudimentary Wolffian tubules, which develop in con- 

 tinuity with it. In the succeeding stages the lumen of 

 the duct gradually extends backwards and forwards. 



