ox THE FuETAii MEMBL'ANES (JE CHELuMA. 31 



a sieaiii for many more sections, a>; the .simple incision of the alhmtois 

 extends considerably further postericjrly than the most posterior 

 point of the sero-amniotic connection. Finally, however, the 

 allantoic cavitv is continued across. As the incision is deeper in the 

 iimer liml) of the allantois than in the outer, the allantoic cavity first 

 becomes continuous near the external surface and then u'radually 

 extends tcjward the inner surface. 



h. Trknujx Jaitonieu.s, 



As in Clemmys. the allantoic blo(jd-\essels group themsehes 

 into two sets : the antei'ior (or the right) and the posterior (or the 

 left), while the allantois is still a small vesicle (Fig. 27 PI. l\.). 

 AVhen the allantois has advanced simiewhat in its de\'elopment, it 

 presents the shape represented in Fig. 72 (PI. VIII.). This cor- 

 responds to Fig. 67 of Clemmys but presents some important 

 differences. The allantois consists here of two lobes marked off 

 from each other bv two constrictions. One of these is just l)ehind 

 the eve and the other is directly opposite the first on the opposite 

 side of the ^•esicle. Pnlike Clemmvs, both these (/«jnstrictions are 

 pi'odiiccd in the same way. That is, the line along which each 

 set of blood-vessels jwisses from the inner to the outer liml) of the 

 allantoic Aesicle is left behind in its growth, and the parts on each 

 side of" the >anic hni' growing faster and meeting each other sotjn 

 pnjdiice a mesenterv-like f)ld slingiiig these blood-vessels. In 

 tjther uords, Ixjth the constrictions of Tri onyx are of the same 

 nature as the anterior constriction of Clemmys (Fig. 67). The 

 posterior constriction of Tri onyx is not well-marked in Clemmys: 

 it corresponds to the shallow notch caused by the posterior or left 

 set of blood-vessels. ( )n the contrary, that corresponding to the 



