376 



has a floor composed of a thin layer of ectoderm. The regions 

 marked 3 and 4 in the figure indicate the proximal parts of the two 

 embryos that are designated by these numbers, and these are slightly 

 separated from each other by the depression whicii lies just to the 

 left of the point marked by 4. lu passing further to the right in 

 the series, one can trace the bifurcation of the bud from its beginning 

 at the fork out to the tips of the embryos, which gradually diverge 

 from each other. In Fig. 9 the character of each embryonic rudiment 

 is most clearly shown. The embryo proper is composed of a thick 

 plate of ectoderm that is intimately nnited to the entoderm lying above 

 it. On the lower side the plate of ectoderm is directly continuous with 

 the thin amniotic portion of the tube. In speaking of above and 

 below in this connection, it must be kept in mind that on account 



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Fig. 9. Photograph from the same series, a little further to the right, and 

 showing the sections of embryos 3 and 4. 



of the inversion of germ layers the entoderm lies above the ectoderm 

 and forms the outer surface of the vesicle. 



Figure 10 is taken from a series of transverse sections of a 

 vesicle in wjiich the relation of the four embryos to one another and 

 to the various parts of the vesicle is well shown. Embryos 3 and 4 

 are the product of the right-hand primary bud, and are entirely 

 separated from each other. Embryos 1 and 2 have arisen from the 

 left-hand primary bud. The section passes through the anterior tip 

 of these two embryos, and consequently they are confined within the 

 cavity of the proximal part of the primary bud. In the sections 

 which lie closer to the base of the vesicle, embryos 1 and 2 eacli 

 have separate amniotic cavities. 



