300 
E. W. MACr.KIDE. 
now examine a series of transverse sections through an 
embi-yo in wliich the diminution in diameter is only sliglit, 
and in whicli a good part of the nerve-plate is still uncovered, 
we shall see the first stages in the formation of the mesoderm 
and the notochord. As my contention that in the formation 
of its mesoderm Amphioxus resembles Balan ogloss u s is 
based on an examination of this stage, and as Cerfontaine 
appears to have completely missed it, I have thought it wise to 
figure in detail five sections from such a series. Figs. 11 a, h, 
c, d, and e (PI. 2) represent these sections. In fig. 11 a, the 
most anterior, we see the nerve-plate consisting as yet of a quite 
horizontal platform of columnar ectoderm cells. In the centre 
are two clear cells (or), which I suspect are part of the 
rudiment of the eye-spot which is found in this position in 
later larvm. The archenteron shows at its dorso - lateral 
angles two pouch-like outgrowths {coll.). The form of these 
pouches and the sharp angle at which their cavities unite 
with the cavity of the archenteron negatives any idea that 
they could be due to a simple process of folding of the 
archenteric wall; they are clearly due to an active process 
of proliferation which takes place at this position in the 
endoderm. In fig. 11 h, which is only two sections further 
back, we see that the nerve-plate is no longer flat but gently 
arched inwards, and the beginnings of the two ectodermic 
folds (/), which are later to meet above it, are seen at its 
sides. In the archenteric wall we observe the same pouches 
that we remarked in the more anterior section, but above 
them and situated nearer the mid-dorsal line we find another 
pair of similar pouches {fr.). In fig. 11c, which is only two 
sections further back still, the ectodertnic folds have almost, 
but not quite, met above the nerve-plate, and the first and 
more ventral pair of pouches of the archenteric wall have 
disappeared, but the second or more dorsal pair persist and 
continue through half a dozen sections more, always open to 
the gut, though this opening is narrower in one place than 
it is in front or behind. Now the first or more ventral pair 
of pouches give rise to the first pair of somites, whilst the 
