ON THE FOETAL MEMBRANES OF CHELONIA. 



11 



body o f t h e embryo itself, connecting' the amniotic sac 

 with the exterior. A reference to Fig".s. 4-7 (PI. I.), will make 

 the growth of this posterior tube clear. In Fig. 4, the folds have 

 extended slightly beyond the posterior end of the embryo. Beyc^nd 

 this point, they snddenly come near each other, and being diminished 

 very much in width, their continued growth backward produces a 

 tube (Figs. 5 and 6). It will be seen that the extreme posterior point 

 always presents a horse-shoe shaped outline, as it did when growing 

 over the body of the embryo itself. Fig. 7 shows the stage of the 

 greatest development of this tube in my possession. Tn three 

 embryos of this stage whose lengths are 8, 8, and 8 \ millimeters, the 

 length of the posterior tube of the amnion is respectively 6, 8, and 7 ^ 

 millimeters. The posterior opening is s(^me distance beyond the 

 edge of the vascular area. 



The sections of this tube show that the relations of the 

 different layers are in all essential respects exactly as in that part of 

 the amnion )»roper enclosing the einbryo as shown in Figs. 39 and 40 

 (PI. v., from the embryo given in Fig. 5) of which Fig. 39 is froni the 

 anterior part of the tube near the embryo and Fig. 40 from about the 

 middle of the tube. In the surface view, there is often seen a streak 

 along the median line of the tube, which is shown by the sections to 

 be a thickenino- on the floor of the tube. The structure of the similar 

 tube in Tri onyx is gi\en in a more enlarged scale in Figs. ô3-r)o 

 (PI. VI.). 



What the function of this remarkable tube connecting the am- 

 niotic sac with the exterior is, -^whether it has any active function at 

 all or is only of the nature of a remnant (-»rgan, I am unable to tell. 

 I think it probable that it serves for conducting into the amniotic 

 sac the nutritive matter from the Avhite, with whose gradual 

 disappearance from over the embryo the backward grow^th of the . 



