THE FORMATION OF THE EATERS IN AMPHIOXUS. 323 
neural plate is progressively formed by the closing of a 
slit-like blastopore — an idea entirely negatived by the 
examination of those more primitive developmental histories 
which we have already described. 
Before passing on to consider the development of the 
Amniota there is one case which demands special attention, 
because a great deal of weight has been laid on it. That is 
the description of the development of the Gymnophionan 
genus Hy pogeophis by Brauer (8). Here there is a compara- 
tively large yolky egg with a germ-disc which alone segments, 
though nuclei are found scattered throughout the unseg- 
mented yolk. The superficial layer of the germ-disc becomes 
differentiated as a well-marked columnar epithelium from the 
underlying rounded yolky cells. At one point a raised rim 
makes its appearance, and here according to Brauer an 
inflexion of the columnar epithelium takes place, leading to 
the formation of a sac whose roof is columnar epithelium, 
whilst its floor is composed of rounded yolky cells. At the 
same time irregular spaces interpreted as a “segmentation 
cavity ” have appeared within the yolky cells themselves. In 
the next stage the cavity formed by invagination has broken 
through into the “segmentation cavity,” and so a quite 
roomy archenteron is formed. In the meantime from the 
opposite site of the germ-disc the ectoderm has swept 
completely round the egg, and so the blastopore is outlined. 
The dorsal wall of the archenteron consists partly of 
cylindrical cells and partly of rounded cells, and Brauer 
maintains that the part composed of cylindrical cells 
can be distinctly delineated from the part made of rounded 
cells; the part consisting of cylindrical cells becomes then 
separated from the rest by the growth under it of the sides 
of the yolky layer which constitutes the floor, the sides and 
front part of the roof of the archenteron, and this sheet of 
columnar cells is entirely used up in forming the lateral 
sheets of mesoderm and the notochord, and in this way the 
epithelium of the alimentary canal comes to be composed 
entirely of “ vegetative ” or lower layer cells. Founding on 
