POLYEMBRYONIC DEVELOPMENT IN TATUSIA 605 
bryos. The ‘right-lateral’ (II) and ‘left-lateral’ (IV) embryos 
lie at the acute angles of the diamond, while the ‘dorsal’ (III) 
and ‘ventral’ (I) embryos occupy the obtuse angles. The same 
condition prevails in two specimens but slightly older than this 
one; hence we may conclude that it is the normal Se 
of the secondary buds. 
It is certain from the evidence that Embryos I and II are the 
product of the right-hand primary bud, while Embryos III and 
IV have come from the left-hand primary bud. It might appear 
from an examination of figure 2 that the secondary buds at ITI 
and IV had alone come from the two primary buds of such a 
specimen as No. 247 (fig. 1), while those at III and I had 
arisen de novo from the dorsal and ventral sides, respectively, 
of the ectodermal vesicle. But the entire genesis of these sec- 
ondary buds argues against such an interpretation, as will be- 
come clear after a complete account of their development has 
been given. Nevertheless, it is an interesting fact, and one I 
believe not to be without significance, that the secondary buds 
II and IV occupy positions on the periphery similar to those of 
the two primary buds. Furthermore, it is probably correct to 
regard Embryos I and III as having arisen as outgrowths from 
the left sides (assuming that the observer is situated in the cen- 
ter of the free surface of the blastocyst) of the right and left 
primary buds, respectively. 
The oval figure in each bud indicates the area over which the 
ectoderm and entoderm have come into intimate contact or have 
fused, while the solid line within each oval area shows the posi- 
tion and extent of the primitive groove. The head of the future 
embryo will, in each case, be directed towards the center of the 
vesicle. 
The plane of the sections is indicated by the broken parallel 
lines, which also show the position of the four sections from 
which the photographs of plate 7 were made. 
The section passing through the plane e to f will be taken for 
detailed account of the different parts of the chorionic vesicle. 
The outer membrane of the chorionic ectoderm (fig. 23) shows 
signs of breaking away. It has usually disappeared before this 
