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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 Eabl, serve to distin- 

 guish pre-otic and post-otic meso- 

 dermal segments. The evidence 

 which I have obtained may be sum- 

 marized as follows : Cross sections 

 of embryos in early stages of devel- 

 opment leave no doubt that the 3d 

 somite, as its topographical relations 

 to chorda, dorsal aorta, epi branchial 

 line, and dorsal wall of alimentary 

 canal show, is composed of only dor- 

 sal mesoderm. Figure A represents 

 a cross section in the region of this 

 somite from an embryo with 28 

 somites (compare Plate 3, Fig. 13). It is seen that a well marked 

 cavity (myocoel), surroimded by a single layer of epithelial cells, may 

 be distinguished.^ 



1 Both van Wijlie ('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 tlie 

 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 ; hrs. vsc. 1, first visceral pouch ; cd., chorda dorsalis ; ec'drm., 

 ectoderm ; en'drm., entoderm ; my'coel., myocoel, enlarged ventrally to form a sclero- 

 tome vesicle ; tb. n., neural tube. 



Figure A. 



