THE FORMATION OF THE LAYERS IN AMPHIOXUS. 299 
contrast between the ectodermal and endodermal nuclei at 
the lips of the blastopore, but in the more ventral section the 
endodermal nuclei protrude beyond the lip somewhat, whereas 
in the more dorsal they are pushed inwards by the over- 
growth of ectoderm. From these figures we may conclude 
that the point y like the point x is really an arc of considerable 
lateral extent — in fact, of much greater extent than x. We 
should probably be justified in dividing the whole circum- 
ference of the blastopore into a dorsal and a ventral arc; by 
cell division in the former gastrulation is initiated, by cell 
division in the latter the blastopore is closed. Text-fig. 1 
represents approximately my view of the steps by which 
gastrulation proceeds in Amphioxus. It is impossible, of 
course, to define the limits of the new endoderm and of the 
original endoderm which formed the flat surface of the hemi- 
spherical blastula, because, as we have seen, mitoses take 
place in the original endoderm and thus it increases in extent. 
Tlie text-fig. gives the most probable view of their relative 
extent. On account of this cell-division in the original endo- 
derm its cells become reduced in size, and at the conclusion 
of gastrulation the archenteron is lined by a layer of cells 
which are all of approximately equal size. 
The Formation of the Notochord and of the Mesoderm. 
We have seen that the blastopore becomes covered in by 
the meeting above it of two folds of ectoderm, which arise 
at the sides of the nerve-plate. These folds first arise in the 
hinder part of the embryo and first meet there, and they then 
grow gradually forwards, but for some time the anterior part 
of the nerve-plate is uncovered. During this time the embryo 
is growing in length and pari passu diminishing in diameter. 
The only possible explanation of this phenomenon which I 
can make to myself is that the yolk-granules are being used 
up, and that coincidently the cells are multiplying all over 
the animal, but especially towards the hinder end. If we 
