POLYEMBRYONIC DEVELOPMENT IN TATUSIA 613 
such as we have just considered the vesicle grows rapidly in 
size, quickly filling up the entire lumen of the uterus. The first 
noticable change in growth affects the free or distal portion of 
the wall, and the chorionic vesicle soon becomes bulb shaped 
(fis. %). 
The expansion and growth of the wall of the blastocyst grad- 
ually carries distally the common amniotic vesicle, which by this 
time has ceased to grow. As a result, the embryonic rudiments 
are gradually separated from the amniotic vesicle, and their or- 
ganic connections with the vesicle are drawn out into small 
tube-like structures, the amniotic connecting canals. These 
canals lie against the under or inner surface of the entoderm, 
and each consists of an inner layer of ectoderm, surrounded by 
a layer of mesoderm. 
In plate 11 are shown five sections, taken at different levels, 
from a specimen in which the amniotic vesicle was just begin- 
ning to be drawn away from the embryonic rudiments, through 
the expansion of the wall. The sections are not quite transverse 
to the axes of the four embryos. 
The section represented in figure 80 cuts the tip of the chori- 
onic vesicle and passes through the lower portion of the common 
amniotic vesicle. The anterior parts of three of the embryos 
are seen in the section. These are the dorsal, right-lateral, 
and ventral embryos. The position of the other embryo, or 
the left-lateral, is indicated by the evagination at the left side 
of the vesicle. 
In figure 81, which is three sections further down, the same 
relation with reference to the embryonic rudiments still exists, 
but the section passes through the extreme lower limit of the 
amniotic vesicle, and the right-lateral embryo becomes entirely 
separated from the others. 
In figure 82, which is five sections lower down on the chorionic 
vesicle than figure 81, the dorsal and right-lateral embryos are 
both free from any connections with the amniotic vesicle. In 
section each embryo appears as a section of a tube. The chief 
interest in this section lies in the condition of the ventral and 
left-lateral embryonic rudiments. Only the anterior tips of 
