FROM TWELVE TO THIRTY-SIX SOMITES 197 



The main part of the mesonephros is thus between the twen- 

 tieth and thirtieth somites. In the anterior half of this region 

 three or four rudiments of tubules are formed in each somite by 

 the seventy-second hour. Subsequently five or six tubules are 

 formed in each segment between the twentieth and thirtieth. 

 Tubules are formed first from the ventral portions of the neph- 

 rogenous tissue (see Fig. Ill); those formed later arise from 

 the unused portions. There is no evidence that they ever arise 

 in any other way. The tubules may thus be divided according 

 to the time of origin into primary, secondary, tertiary, etc., sets, 

 but there is no morphological or functional distinction between 

 the successive sets. (See Chap. XII.) 



The collection of tubules causes a projection or fold on each 

 side of the mesentery into the body-cavity, known as the Wolffian 

 body, the detailed history of which is given in Chapter XII. 



In conclusion it should be noted that the most anterior tubules 

 of the Wolffian body possess peritoneal funnels like the pronephric 

 tubules. Thus in an embryo of 30 somites I have noticed open perito- 

 neal funnels in the eighth, ninth, twelfth, thirteenth, fourteenth, fifteenth, 

 sixteenth, and seventeenth somites. It seems quite certain that the 

 last of these belong to the mesonephros, though the most anterior are 

 undoubtedly pronephric rudiments. In the eighteenth, nineteenth, 

 twentieth, and twenty-first somites, small depressions of the peritoneum 

 were noticed opposite tubules, but not communicating with them. 



The Vascular System. Soon after the thirty-third hour the 

 heart begins to twitch at irregular intervals, and by the forty- 

 fourth hour its beatings have become regular and continue unin- 

 terruptedly. The contraction proceeds in the form of a rapid 

 peristaltic wave from the posterior to the anterior end of the 

 cardiac tube, and the blood, already present, is forced out in 

 front. Through the aortic arches it reaches the dorsal aorta 

 which distributes part to the body of the embryo, but most of 

 the blood enters the vascular network of the yolk-sac. It is 

 returned to the heart by various veins in the yolk-sac and em- 

 bryo, and recommences the circuit. 



The development of the vascular system will be more readily 

 understood if we preface the account with a brief description of 

 the anatomy of the system early in the fourth day (Fig. 115, 

 cf. also Figs. 135 and 136). 



The heart consists of four chambers, viz., the sinus venosus, 



