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.’ The difference 
between the two buds exists throughout their entire length. 
Thus in 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. 
