THE FORMATION OF THK LAAGERS IN AAIPHIOXUS. 293 
place, and of wliicli tlie mitosis shown [k) is evidence. A 
slightly later stage is shown in PI. 1, fig. 2. In this embryo 
the process of invagination has commenced. At the point 
we may recognise the same abrupt transition in the size of the 
cells which we remarked at the same place in the preceding 
stage. Here an nnmistakable mitosis can be seen. The im- 
pression conveyed to the mind is that the new cells are added 
to the flat side by the process of cell division which has begun 
at .T, and that these have exercised a lateral strain to which 
the fiat side has yielded by bending inwards. The convex 
side has become moi-e convex than in the preceding stage, 
and its cells more numerous. The cell-division at x has 
therefoi'e probably added cells also to the convex side, but 
there is not the slightest evidence that any cell orginall}' on 
the conve.x side has passed on to the flat side or vice versa. 
In fig. 3 we have a median sagittal section of a still later 
stage. AVe again notice a mitosis at the })oint a;. We obtain 
further evidence that the cell division which is taking place 
at this point does not result in a passage of cells from the 
convex side to the invaginated side of the embryo, because 
the cells adjacent to this point on the two sides are exceed- 
ingly different from one another. Those belonging to the 
convex side are smaller than those on the invaginated side, 
and have fewer volk-granules. The new cells on the convex 
side are remarkalde for their rounded form ; cells of this 
shape have been noticed in the neighbourhood of the blasto- 
pore by several authors ; the shape seems to be indicative of 
the state immediately following division when the 
centric force of the new nucleus is at its maximum strength 
(v. TheePs beautiful figures of the development of Echino- 
cvainus pusillus [33]). At the point // the large clumsy 
cells are continued some little distance beyond the lip of the 
invagination on to the convex surface. 
A still later stage is represented in fig. 4. As compared 
with fig. 3, Ave note that the invagination has become deeper, 
and that a decided difference in staining property 
between the nuclei belonging to the convex side 
