196 BULLETIN MUSEUM OF COMPARATIVE ZOOLOGY. 
region. So far as I know, hitherto no explanation of this phenomenon 
has been suggested, 
That which I have regarded as the more serious of the objections 
made by Rabl (89 and ’92), viz. that the pre-otic segments are not 
morphologically comparable with trunk somites, inasmuch as they do 
not show a differentiation into myotome and sclerotome, may be met by a 
denial of the statement, so far as it applies to the 3d somite of van 
Wijhe.! I have followed the development of this somite through closely 
connected stages of development, until it becomes converted into the muse. 
rectus posterior and assumes relation with the eye, in order to determine 
whether in its development it exhib- 
its those marked differences which, 
as stated by Rabl, serve to distin- 
un tbn. guish pre-otic and post-otic meso- 
dermal segments. The evidence 
which I have obtained may be sum- 
marized as follows: Cross sections 
un OCUYM, 
PR? s 
.mycoet. opment leave no doubt that the 3d 
of embryos in early stages of devel- 
somite, as its topographical relations 
00.0. , : 
to chorda, dorsal aorta, epibranchial 
line, and dorsal wall of alimentary 
en drm, 
canal show, is composed of only dor- 
a .örs.vscs. sal mesoderm. Figure A represents 
a cross section in the region of this 
FIGURE A. 
somite from an embryo with 28 
somites (compare Plate 3, Fig. 13). It is seen that a well marked 
cavity (myoccel), surrounded by a single layer of epithelial cells, may, 
be distinguished.? 
1 Both van Wijhe (*82) and Killian (’91) have affirmed a differentiation of head 
segments into myotome and sclerotome. 
2 That the epithelial walls of the cavity (Fig. A) are not continuous with the 
two layers of the lateral plates is due to the obliteration of these two layers caused 
by the great development of the first visceral pouch. 
Fig. A. Cross section of a Squalus embryo in the region of van Wijhe's 3d 
somite and encephalomere IV. X 240. The dorsal nature of this mesodermal seg- 
ment is attested by its relations to dorsal aorta and wall of alimentary tract. At this 
stage (Acranienstadium) the region of proliferation of mesenchyma is seen to be a 
definite one, and to correspond in its relations with the sclerotome of trunk somites. 
ao. d., dorsal aorta; brs. usc. 1, first visceral pouch; cd., chorda dorsalis ; ec’drm., 
ectoderm ; en’drm., entoderm ; my'cal., myoceel, enlarged ventrally to form a sclero- 
tome vesicle; tb, n., neural tube. 
