LIFE-HISTORY OF XENOPUS LAVIS, DAUD. 803 
the elevations on the upper contour of fig. 8, Plate 1, near the middle line as the medul- 
lary folds, otherwise it would be ditticult to make out their position in the posterior view 
of this stage. They flatten out and disappear towards the blastopore. On the outer side 
of each medullary fold is a band of pale ectoderm cells which can be seen at the pomts 
a and b to be slight ridges. These bands of proliferating ectoderm cells meet at a point 
just anterior to the future anal perforation, marked at this stage by a small pigmented 
area at the hinder end of the closed and elongated blastopore. As these bands lie ex- 
ternal to the medullary folds, it is certain that at this stage the medullary folds do not 
enclose the blastopore or meet behind the anal opening. The archenteron is now com- 
pletely closed to the exterior, as for a short time the blastopore is quite closed ; there is 
noanus. ‘The anal opening is formed in a few hours after this stage has been passed 
through. The neurenteric canal and postanal gut persist for a long time; they are 
present at the stage shown in fig. 14. The dark area (fig. 84) anterior to the medullary 
folds, and extending a short distance back along the sides of the fore-brain region, is a 
region covered with modified, deeply pigmented ectoderm cells, the greater number of 
which are destined to become secretory epithelium cells. ‘The median ventral portion 
of this area is the precocious rudiment of the cement organ (‘‘ Sucker”) ; the lateral dorsal 
portions contribute to the formation of the frontal gland (‘‘ Stirnknospe,” v. Kupprmr, 
“Stirnstreifen,” Hinsperc). This area corresponds in its position and relations to 
other parts exactly to O. ScHuurzr’s “ Sinnes-platte,” Morean’s “Sense-plate.” It 
is not to be regarded as a lateral extension of the medullary plate (Moreay, ’97, 
p. 57). It is a part of the general ectodermal covering without any obvious 
connection with the central nervous system and formed by differentiation in the 
cells of the superficial epidermic layer of the ectoderm only. The deeper nervous 
layer of the ectoderm is not modified and there would seem to be no justification for 
the term “ Sinnes-platte.” 
The portions of the ectoderm from which: the epithelium of the nasal pits is derived 
are enclosed by or are possibly included in this dark area; it is not possible to identify 
them at this stage. 
The medullary folds have arched over and their edges have met along their whole 
length in the next stage figured (fig. 9). The ectoderm is raised as a gentle ridge over 
the closed neural tube, and there is a line of deeply pigmented ectoderm cells along the 
median external line of junction of the folds, and at the bottom of the shallow groove 
formed by the rounded edges of the folds. The anterior end of this pigmented groove 
marks the position of the neuropore. A short distance posterior to the neuropore, the 
pigmented groove is intersected by a crescentic band of pigmented ectoderm, the early 
rudiment of the frontal gland. Below the position of the neuropore is a large pigmented 
patch of ectoderm, the rudiment of the cement organ. It shows some indication of a 
paired nature in the presence of a more deeply pigmented patch at each end of the 
transversely elongated area. This figure (9), compared with fig. 8a, shows that the greater 
part of the pigmented area in front of the medullary folds at the earlier stage becomes 
