166 RITTER. [Vox. XII. 
letters are the very beginnings of the folds. In neither of 
these figures, nor in fact in any of the numerous similar ones 
that might be given, does it appear as if the peribranchial sacs 
begin as evaginations from the primitive vesicle. It does not 
appear here as if the peribranchial sacs are initiated by the 
active growth of the portions of the primitive vesicle walls 
which are to enter into them, but, as already said, by the 
growth of the partitioning folds. On the other hand, sections 
of a very early stage in the development of the sacs show that 
in one instance, at least, there takes place an active growth of 
the Anlage of the sacs themselves, resulting in well-defined 
evaginations. (See Figs. 20 and 21, 67s.a., Pl. XIII.) These 
figures are drawn from sections of a bud presenting an earlier 
stage in the development of the sacs than that represented by 
Fig. 16, and so it might be assumed that the sacs of the latter 
individual had their beginnings in such evaginations as those 
shown in Figs. 20 and 21, even though all indications of them 
are by this stage completely lost.? 
But if I am right in supposing the angles #.f. in Fig. 16 are 
the beginnings of the folds, it is not quite clear how such a 
condition as that presented by this bud would be related to 
one in which the well-defined evaginations of Figs. 20 and 21 
were present. The only way of harmonizing the two condi- 
tions, so far as I can see, is to suppose that the sacs begin 
with the evaginations, and that these grow rapidly in width, 
but very little in depth, till they extend over nearly the whole 
of the sides of the primitive vesicle. Posteriorly they extend 
ultimately so far as to be separated only by the angle in which 
the intestine (zz¢., Fig. 16) develops, and anteriorly the angles 
are almost obliterated, but the sacs do not approach each other 
so closely as behind, but remain separated by the thickened 
area (end., Figs. 16 and 17), which will give rise to the endostyle. 
Thus we should have the condition presented by Fig. 16 asa 
more advanced stage of development derived directly from the 
earlier distinctly evaginated stage. And the fact that the 
hypophyseal duct and intestine are both begun in the bud 
1] have since seen in a whole bud an evagination from the inner vesicle, at 
least on one side, that in all probability corresponds to 47.5.4. in Figs. 20 and 21. 


