3,5 ( HTROTA: OX THE SERO-AINIXIOTTO rOXXEOTIOX, 



themselves, in ordei- tliat the influence of the sero-Mmniotic connection 

 iipc^n them m.'iy he l)ei"ter understood. 



Wlien nt the end of the tliird dny or early in the fourth day, the 

 allantois ap))ears on the rio'ht side of the emhryo. hetween the serous 

 envelope and the amnion, it is a discoidal sac with circnlar outline. 

 At the end of the fifth day we can distinü'in'sh in a surface view the 

 riüht and left allantoic arteries and one larsfe allantoic vein in the 

 allantois (Fia*. 7). The right artery, which is always i)ifiircate in the 

 inner limhof the allantois, supplies the posterior part of the allantois. 

 while tlie vein is divided into two or three main hi-anches in the 

 outer limh. The left artery is smaller than the otlier two \cssels, and 

 is destined to supply the anterior part of the ;dlantois. 



From this stage the allantoic vein begins to indent the pre- 

 viously circular maraiu of the allantois at the point where the 

 \ein ]iasses fr<^m its outer to its inner lind). The obstruction by 

 the vein restrains and retards the growth of the allantois at this 

 point, althougli it does not stop it entirely. Contiguous to this 

 point, therefore, the allantois grows fister than at the ])oint itself 

 and thus an indentation is produced. With the growth of the allan- 

 tois, the indentation becomes of great depth in later days, and the 

 lobe of the allantois which lies in front of it comes gradually to 

 overla]) the lobe posterior to it, beginning from tlie a])ex of the 

 indentation (Figs. <S, 9, and 10). 



The riglit and left arteries also have a tendency to restrain the 

 free gr<^wth of the allantois at tlie points Avhere tliey ]iass from the 

 inner to the outer limb. The eifect is, however, (piite trivial in 

 comparison with that of the allantoic vein. 



It is noteworthy that there is an intimate relation between the 

 allantoic vein and the sero-amniotic connection. The allantoic vein 

 always restrains the free growtli of the allantois at a ]ioint a little fo 



