Development of the Fourth Day 255 



the two vessels has been entirely accomplished, 

 the efferent hepatic vessels shift their position 

 so as to open directly into the posterior cava, 

 instead of into the meatus venosus (Fig. 78) ; 

 and, before the time of hatching, the part of the 

 meatus venosus between the heart and the liver 

 becomes obliterated, so that all of the blood 

 that flows into the posterior end of the liver, 

 through the portal vein, empties into the pos- 

 terior cava through the hepatic vein (Fig. 79). 



One thing that is sometimes confusing in 

 trying to trace the development of the circu- 

 latory system by studying a series of figures 

 like those in the text, is the variation in the 

 relative sizes of the different vessels ; if this 

 feature be kept in mind, it may help to make 

 the relationships between the successive stages 

 more evident. 



The more important features in the develop- 

 ment of the blood vessels, from their earliest 

 beginning to the adult condition, have now 

 been described ; and it may help to make the 

 whole subject more clear if a brief description 

 be given of the course of the circulation during 

 the latter part of the period of incubation, 

 together with the changes taking place at the 

 time of hatching. 



