THE EOEMATION OF THE LAYERS lA’' AMPHIOXUS. 
295 
nuclei is seen, but fig. 5 a deserves our special attention 
because of the small rounded cells which are seen on both 
sides of the point x. As may be clearly seen by reference to 
Cerfontaine’s paper, it was on sections like this that that 
author based his conclusion that ectodermal cells were being 
added to the vault of the archenteron ; we have already sug- 
gested that the rounded form is a passing phase following on 
mitosis; now our figure shows that the rounded cells which 
are added to the arch enteric wall have vesicular 
nuclei, Avhilst those added to the ectoderm have 
more deeply staining' nuclei; all question, therefore, of 
there being any ^‘inflection” of ectoderm cells is settled in 
the negative. I have spoken of “ the point ” x when referring 
to a median sagittal section of the gastrula ; the growing 
point is really, however, a horizontal arc, and appears in 
several sections on each side of the median one; fig. 5 h, 
which represents a lateral section in which the cells at the 
dorsal lip of the blastopore are quiescent, lies four or five 
sections to one side of the median one. The ectoderm cells 
immediately beyond the growing point in fig. 5 a are seen to 
be specially tall and columnar. This is the first indication of 
the nerve-plate [n.'p.), and is of the greatest, importance for 
the decision as to the orientation of the blastopore. It is also 
seen in the older stage represented in fig. 6 («.p.) ; from these 
two figures we may conclude that the diameter of the 
wide-open blastopore is at right angles to the long 
axis of the nerve-plate, and this conclusion seems to me 
to settle in the negative the question of the closing of the 
blastopore by a concrescence proceeding from in front back- 
wards. For such a theor^q whether advanced by Hatschek, 
Legros, or Cerfontaine, has always assumed that the nerve- 
plate is formed pari passu as the blastopore closes, and this 
could only be possible if the long axis of the nerve-plate 
and the diameter of the blastopore coincided in 
direction. In fig. 6 we can see sevei'al mitoses in what even 
Cerfontaine would admit to be the endodermic portion of the 
archenteron, and, what is most interesting, one of the cells 
VOL. 54, PART 3. — NEW SERIES. 22 
