;>5,S HIROTA: ON THE SERO-AMXIOTIC CONNECTION, 



fio-ure. The ulbumeu-sac .separated from the aiiiiiioii foi' the most 

 [»art hy the intervening yolk-sac, «ends out o\'er the latter a hodow 

 cave-Jike diverticulum towards the amnion (see Figs. l-S and l<)j, 

 with which it c<jnies in ct)ntact. The contact plate at the lu^ttoni 

 of this diverticulum is the sero-amniotic connection (S. A. C, 

 Fio-. IS and K! : sections in Figs. 80 and <S1). In fact, it is 

 the presence of the sero-amni(jtic connection that lias j)roduced the 

 peculiai" cave-like diverticulum. Let us now turn to the allaubjis 

 itself. '!<) understand the complexities arising in this membrane, 

 it is necessarv to bear clearly in one's mind the facts whicli liave 

 been brought ont in tlie preceding pages ; (1) that the allantoic vein 

 in passing from tlie inner to the outer limb of the allantois causes 

 an indentation in the margin of the allantois, retarding its growth 

 at this point, and the lobe of the allantois in front of the indentation 

 comes t(j overlap rhe lobe posterior to it, the area of overlapping 

 being continuously increased with age; (2) that a terminal part of the 

 left allantoic artery and a branch of the right allantoic artery produce 

 respectively a minor indentation in the margin of the allantois. We 

 nmst therefore kn<j\v well how these blood\'essels run in the allantois. 

 This may be learned by com|jaring carefully Fig. 14 and 15. First 

 notice in Fig. 14 where all the allantoic vessels come out together 

 Irom the umbilicus (for the exact position of which see also Fig. 17), 

 tlie allant(3ic vein runs from the umbilicus to the point F (Fig. 15), 

 along the inner limb ; then from the point F gradually ascends along 

 the line FE to the outer limb on which it: emerges '<^^t the point 

 E. The line F E nuist therefore be regarded as that along which 

 interference witli growth takes ])lace, and the shaded areas F D E, and 

 EGM DE (Fig. 15, for the present disregard the area ABC) together 

 represent the area of overla{)ping of the two allantoic lobes, or in 

 other words the extent of the interallantoic septum. The reason 



