304 
E. AV. MACCRIDE, 
cavifc}". By the forward growth of the nerve-plate the neuro- 
pore region has been carried forwards, so that in the section 
figured it is no longer seen, and not only have the ectodermic 
folds met above the nerve-plate, but the nerve-plate itself 
has been bent into a nerve-tube. The notochord and the 
anterior portions of the collar-cavities have shared in this 
forward grosvth, aud these last are seen above the archenteron 
at the sides of the notochord. Turning our attention now to 
the archenteron we observe that a right thiu-walled vesicle 
and a left thick-walletl one are in process of being shut otf from 
a central thick-walled gut, but that there is still a narrow seam 
Text-fig. 2. 
Illustrating the formation of the mesoderm in Amphioxus. H. Head- 
cavity. c. Collar-cavity. T. Trunk-cavity, ne. Neurenteric canal. 
of communication between right and left moieties. A little 
later these are completely separated from the definitive gut 
and from each other. The relationship to one another of 
head, collar and trunk-cavities is schematically represented in 
text-fig. 2, in which an Amphioxus embryo is supposed to 
be seen from the side. The embryo is supposed to be trans- 
parent, and the formation of the head-cavities is antedated so 
as to make it contemporaneous with that of the collar and 
trunk-cavities. 'I’he portion of the gut, however, from which 
the head-cavities are eventually formed can be clearly seen in 
even young embryos, such as that figured in figs. 11 a-d. It 
projects in front of the region from which the collar-cavities 
are eventually formed. 
