the middle lobe from the larger half of the allantoic vesicle are caused 
in two different ways. The anterior constriction is due to the follow- 
ing circumstances: the line along which the right set of allantoic 
vessels runs, can not on account of their presence keep up in its 
growth with the rest of the vesicle and is necessarily left behind. In 
a short time, therefore, the vessels appear to run in a groove; finally 
the groove becomes very deep, the lobes on two sides of it meet again 
and become firmly appressed with each other, so that practically these 
blood-vessels are supported in their course by a mesentery-like fold of 
the allantoic vesicle. This explains the anterior constriction in Fig. 2. 
The posterior constriction is due to the fact that the vesicle finds 
itself unable to spread freely over the embryo on account of the 
sero-amniotic connection. The only thing it can do is to grow round 
the sero-amniotic connection, thus producing a deep incision — the 
posterior constriction — in its outline. There are minor details of 
this constriction which are of some importance in later stages but 
need not be entered into here, as they will be described in my full 
paper. 
13. As this peculiarly shaped allantoic vesicle tries to spread 
itself in the extra-embryonic coelomic cavity, the middle lobe finds 
itself checked in its growth by the sero-amniotic connection which 
obstructs it in front. It is therefore the right and left lobes that 
grow to cover the larger part of the yolk-bag. 
14. When the foetal membranes are completed, they appear as 
in Fig. 3 (Diag. VD. The three lobes of the allantois have entirely 
enclosed the yolk-bag and have met one another on the ventral face. 
They, however, never fuse ‚with one 
another but are permanently separate. 
The seams that separate them are roughly a 
speaking tri-radiate, the center being at x 
* 
.. Sero-Amia 
Conn, 
the anterior end of the yolk-sac slightly x 
to the left (to the observer’s right) of $ 
the median ventral line. The seam that ER 
goes from the center towards the right Sr 
side of the embryo, separates the middle 
lobe (in its front) from the right lobe 
(behind it) and corresponds to the an- 
terior constriction of Fig. 2. At its end, 
runs the right set of the allantoic vessels. 
The seam that goes backward from the Frg3. Ventral View of a Clemmys 
= 5 Embryo mes $i days o/d wh, sf 
center parallel with the axis of the embryo Food! Membrancs, ‘tar Sire. 
