360 Ww. F. R. WELDON. 
differentiated mesoderm. On one side there is a single ecto- 
blast outside the mesoblast, on the other there are two such 
cells, In fig. 32, which is ventral to fig. 31 and separated 
from it by the thickness of two sections, two more mesoblasts 
are present, and the mesoderm extends forwards from these in 
the form of a pair of broad bands, meeting in the middle line 
below the alimentary canal, but not as yet extending into the 
cephalic region. In several series of sections, cutting embryos 
of this stage in various planes, I have been able to recognise 
four large mesoblasts, and four only. Iam, therefore, inclined 
to believe that the number is constant. Outside the meso- 
blasts in fig. 82 are seen two more ectoblasts ; but the definite 
relation of the ectoblasts to the cells of the ventral and lateral 
ectoderm can only be fully realised by the examination of 
superficial sections, parallel to the surface of the ectoderm. 
Such a section is represented in fig. 33, where four ectoblasts 
are seen, each of which is at the posterior extremity of a row 
of ectodermal cells. The nuclei of these ectoderm cells are so 
arranged that they fall with equal facility into transverse or 
longitudinal series. The transverse arrangement is most strik- 
ing in views of the whole embryo, and has already been alluded 
to; but the reality of the relation to the ectoblasts, indicated 
in fig. 33, can hardly be doubted. This arrangement of the 
ectoderm-cells can be traced for a considerable distance along 
the ventral face of the cephalic region ; but those sections in 
which itis best seen must necessarily cut the embryo in various 
oblique planes, and explanation of them would be long and 
tedious ; figures of such preparations have, therefore, been 
omitted. 
The relations of mesoderm and endoderm can, perhaps, be 
more clearly gathered from the lateral longitudinal section 
shown in fig. 384, in which the connection between the tubular 
posterior endoderm and the ventral layer produced by the 
rearrangement of the first-formed anterior endoderm-cells is 
particularly well seen. 
The structure of the remaining organs of the Nauplius may 
be most conveniently considered in connection with the history 
