EARLY DEVELOPMENT OF JULUS TERRESTRIS. 457 — 
The keel is shown in transverse section in fig. 9 a, and 
fig. 20. At the end of the sixth day the keel is still present 
but an alteration is taking place in the cells of which it is com- 
posed. They are no longer round and thick, but are becoming 
elongated in the direction parallel to the surface. At the same 
time they continue to multiply and spread themselves out, so 
as to form two definite layers within the ectoderm (fig. 10). 
These are the splanchnic and somatic layers of the mesoderm. 
The cells of the ectoderm and of the somatic mesoderm are 
still connected, and also the cells of the splanchnic and somatic 
mesoderm. 
On the eighth day the mesoderm extends round a great part 
of the embryo—rather more than half way round. The keel 
has almost disappeared (fig. 11). 
On the ventral surface the cells are no longer flat but have 
assumed a columnar form. Their nuclei are now oval in shape, 
their long axis pointing, as does that of the cells to which they 
belong, towards the interior of the ovum. ‘This is in fact the 
first formation of the ventral plate and is shown in fig. 10. 
While these changes are going on the remnants of the keel are 
disappearing. The mesoderm now becomes thicker on each 
side of the ventral line. This is shown in fig. 21. Both 
layers are concerned in this thickening, and at these points the 
two layers become indistinguishable. Outside the thickenings, 
that is, farther away from the middle ventral line, the two 
layers are closely applied to each other and to the epiblast as 
before. The effect of these changes is that the greater part of 
the mesoderm is now arranged in two parallel longitudinal 
bands along the ventral surface of the embryo; these bands 
being connected in the middle line by a thin portion consisting 
of two layers (fig. 22). Fig. 21 is a transverse section through 
the ventral half of an ovum at this stage. 
The two longitudinal bands now begin to be constricted off 
into the mesodermal somites. The latter are formed from 
before backwards and their position corresponds with that of 
the future segments of the body. The number of somites thus 
formed is eight, corresponding to the eight segments with 
