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and those belonging to the invaginated area is now 
observable. The former stain more deeply, since their nuclear 
sap has to some extent acquired staining powers. The transi- 
tion between lightly staining and more deeply staining nuclei 
takes place exactly at x, but at the lower lip of the blasto- 
pore the lai’ge vesicular nuclei are continued round for a 
short distance on to the convex side of the embryo. I regard 
this difference in staining power as the outward sign of 
the physiological differentiation of the cells into 
ectoderm and endoderm; it is observable all through 
the later stages of gastrulation and during the formation of 
the mesoderm. I conclude that at the point x we have a 
growing centre, comparable to the meristem of a root or the 
cambium of a tree with secondary growth, whence oh the 
one side young ectoderm, and on the other side young endo- 
derm cells are produced. To the appearance of this centre of 
growth I attribute the first impulse towards gastrulation, and 
the push which causes the in-bending of the lower cells should 
on this view be directed not towards the centre of the lower 
surface of the flattened blastula, but more towards its dorsal 
edge. As a result the invagination does not at first involve 
all the cells which will eventually be included within the 
archenteron, but the endodermal vesicular nuclei extend 
beyond the lower lip of the blastopore at y. A mitosis can 
be seen in fig. 4 in the centre of the convex surface, and also 
one at the apex of the invaginated area, and they serve to 
emphasise the fact that division of cells is not by any means 
confined to the growing point x. According to this view the 
ectodermal nuclei become differentiated from a type of 
nucleus which we may call endodermal, and this is in accord- 
ance with what we should expect, for the type of nucleus 
found in an assimilative cell must be the primitive one. 
In figs. 5a and hh we have two sections from a sagittal 
series through an embryo almost of the same age as, or very 
little older than, that represented in fig. 4. Fig. 5 a repre- 
sents a median, fig. 5 h a more lateral section. In both 
figures the sharp contrast between ectodermal and endodermal 
