626 Proceedings of Royal Society of Edinburgh. [sess. 
length. On the other hand, in my own case, and in Mitrophanow’s, 
the anterior divergence of the axis was sufficient to allow of two 
separate neural plates and two neural canals being formed, and 
for the same reason two fore-guts, with complete somatopleuric 
and splanchnopleuric folds, while in the other cases these struc- 
tures were single. 
As regards the condition of the primitive streak, the four cases 
present certain differences ; in Kaestner’s the compound chorda 
passed into a simple streak, which forked at its posterior end ; in 
Hoffman’s the streak was forked at its anterior end, and the two 
limbs were continuous with the mesial stem behind, hut again at 
its posterior extremity there were traces of what seemed to he 
accessory streaks. In Mitrophanow’s case the two streaks fused 
behind, hut the primitive groove of the smaller component was 
continued further hack than that of the larger, from which he 
concludes: “Hass das Zwillingspaar . . . aus zwei ganz selbstandigen 
Keimen entstanden ist, welche in Form von Primitivistreifen noch 
selhstandig waren, wobei der Primitivistreifen des kleineren etwas 
hinter dem des grosseren lag, und letzterer seinerseits zu dem 
ersteren in einem Winkel gelegen war.” In my own example, the 
same from a surface view is apparently the case, only the union of 
the streaks is further forward, and there is a single mesial primitive 
groove behind. A careful study of the sections has convinced me 
that such an interpretation cannot apply, hut that there has been 
from the first a Y-shaped primitive streak. The limbs of the Y 
unite to form the broad flat plate described (fig. 13), from which 
the stem passes backwards, narrowing as it goes, till it has the 
typical form of a normal streak (fig. 14). On this Y-shaped 
streak, however, the primitive furrow has not developed as a 
continuous groove, hut independently on the stem and limbs of 
the Y. On the left limb the furrow has differentiated further 
hack than on the right, till it has almost met the mesial furrow. 
There was no trace of accessory rudimentary primitive streaks as 
in Kaestner’s and Hoffman’s cases. 
In considering the nature of a double embryo, such as the one 
above described, one must distinguish between the question of the 
ultimate origin of the duplicity and that of the determination of 
its anatomical characters. It is in regard to the latter question 
