POLYEMBRYONIC DEVELOPMENT IN TATUSIA 611 



thelium, eventually establishing a placental connection with the 

 extending Trager, thus forming a sort of 'belly-stalk' through 

 which the placental blood vessels run, and into which the rudi- 

 mentary allantois later extends. In their growth from the vesicle 

 the buds do not pass out as four distinct rays from a common 

 center, the common amniotic vesicle, but extend out in pairs, the 

 individuals of each pair retaining a common connection with the 

 ectodermal vesicle. The paired condition is but a further expres- 

 sion of the same relation which was noted in connection with 

 the account of the origin of the secondary buds. 



The earliest phase of this condition is very clearly brought 

 out in one of the specimens of the series. This specimen has 

 almost completely collapsed and is inclined to the left. Hence 

 Embryos I and II are in part folded beneath the wall of the 

 blastocyst, making their study difficult. 



In plate 10 are shown photographs of three sections which 

 pass through Embryos III and IV at different levels. Figure 

 75 is taken across the common bay of the two embryos, about 

 half way between the common amniotic vesicle (remains of the 

 old ectodermal vesicle) and the point of departure of the two 

 secondary buds. The chorionic ectoderm has disappeared and 

 consequently the entoderm is the uppermost layer. It is en- 

 tirely distinct from the embryonic ectoderm. 



Figure 76 represents a section which passes through the point 

 where the secondary buds arise and diverge. The bud on the 

 left contains Embryo IV, and its width is almost twice that of 

 its paired mate on the right, or number III. 7 The difference 

 between the two buds exists throughout their entire length. 

 Thus hi the section cutting the middle of the buds (fig. 77) the 

 difference in size is particularly striking. Each embryo consists 

 of the following parts: (1) the entoderm, which is in contact 

 with the primitive streak mesoderm; (2) the primitive streak 

 mesoderm, which is being proliferated from the ectoderm; (3) 



7 In the preliminary paper (figs. 8 and 9) these embryonic buds were incor- 

 rectly labeled. This was due to the fact that the sections had inadvertently been 

 reversed in mounting them on the slide, a fact not discovered until after plate 10 

 of the present paper had been made up. 



