NOTES ON THE EMBRYOLOGY OF LIMULUS. 
531 
inference justifiable that here, as in Peripatus capensis, 1 it 
closes in the middle, the extremities persisting as mouth and 
anus. 
On a previous page I spoke of the shape of the oral opening, 
and said that it was due to the transference of the mouth. 
The mouth in fig. 5 is in front of the first pair of appendages, 
in the adult behind them. The process of this transference is 
interesting. After the disappearance of the primitive groove 
behind the mouth a depression gradually extends backwards, 
and at the same time at the front the edges rise up and finally 
unite to form a close tube, the stomodseum (figs. 40, 41, 42, 
43), in almost exactly the same way that the neural canal is 
formed in the chick. This seems to me an important point, 
for it shows that the functional mouth is not a strictly homo- 
logous structure throughout the animal kingdom, but that 
in those forms with a stomodeum it has been considerably 
modified in position. Unfortunately we do not know if a 
similar modification exists in Peripatus. The anus (fig. 46) 
is a shallow pit, and at this stage shows no signs of forming a 
proctodseum. 
From this point on it will prove the easiest and possibly the 
best to consider separately each of the tissues or organs without 
attempting to describe the embryo at each stage as a whole. 
Mesoblast. 
In fig. 4 the mesoblast constitutes a broad sheet (fig. 44), 
but between this and fig. 5 a considerable gap occurs in my 
material. In this latter stage it has become separated into 
two broad bands, except at the extremities. It seems probable 
that this separation is effected partly by the rapid growth of 
the epiblast in the ventral region, and partly by a migratory 
movement of the cells. In the region of the mouth it is still 
1 Kennel’s (’ 84 ) recent researches on two South American species of Peripatus 
do not prove Balfour wrong in his interpretation, and until further observa- 
tions are published onP. capensis, I think that his observations, certified to 
by Moseley and Sedgwick, should be accepted. Nor do I think Kennel has 
proved Sedgwick’s ideas “ ungeheurlich,” for Lang on the Planarians and 
Wilson on the Alcyonaria come to the same general conclusions. 
