190 BIJLLETIX: ilUSETJM OF COMPAEATIVE ZOOLOGY. 



median wall migrate iuto the centre of the cavity, and cells bounding 

 the inner wall above and below assume the elongated contour of muscle 

 cells. 



Hoifmann ('94, p. 649) also, while not able to state with definiteness 

 that the anterior cavity is a dorsal or lateral diverticulum from the 

 alimentary canal, i. e. whether it represent a mesoderm segment or a 

 visceral pouch, considers it probable tliat it represents the former, since 

 it is very similar to the succeeding head cavities of van Wijhe. Hoff- 

 mann mentions the migration of cells into the cavity of the somite, but 

 does not specify from which wall they are proliferated. He also states 

 that from the walls of the somite " eutstehen keine Muskelfasern " ('96, 

 p. 256). Against these views of !Miss Piatt and Hoffmann no special 

 arguments have as yet been raised. 



The first somite of van Wijhe possesses the peculiarity of a median 

 stalk connecting the somites of the opposite sides of the body.^ The re- 

 lations of this stalk to the dorsal wall of the alimentary canal, to chorda, 

 and to dorsal aorta have been used as the chief criteria in contending 

 for its dorsal or its ventral nature. The evidences that the first somite 

 represents somatic (dorsal) mesoderm are as follows: (1) Its cells are 

 proliferated from the dorsal wall of the alimentary canal (Piatt, '9P).^ 



1 Such a median connection, however, also appears in the early stages of develop- 

 ment of the " anterior cavities." The connecting stalk of the " anterior cavities," 

 however, as stated bv Hoffmann, never possesses a lumen, as does the median con- 

 necting stalk of the premandibular cavities. 



- Miss Piatt, in her earlier paper ('91, p. 81), states that the mesoderm of the 

 premandibular cavity is formed, at least in part, by a proliferation of cells from 

 the mandibular cavity, while in her later paper ('91*, p. 256) she writes, "The 

 most anterior mesoderm of the head does not take its origin from the mesodermic 

 plates, but from the dorsal wall of the alimentary canal. The mesodermic plates end 

 with the mandibular cavities." The lumen of the connecting stalk, according to Miss 

 Piatt, is, as stated by Marshall ('81), formed secondarily by the fusion of a median 

 with the two lateral cavities. This evidence is interesting, since it bears on the 

 question whether the cavity of the connecting stalk is to be regarded as a part of 

 the archenteron, and would seem to answer this question in the negative. Killian 

 ('91, p. 102), however, finds the connecting stalk a " Sklerotomkommissur," and 

 thus, it is to be inferred, the lumen of the stalk, which according to his account is 

 formed secondarily, not a part of the archenteron. He states (p. 102) : "Erwahnt 

 sei noch, dass zwischen den beiden ersten Mandibularsomiten vor dem vorderen 

 Chordaende und iiber dem Aortensinus ein Mesodermzellenhaufen liegt, der die 

 Sklerotomanteile beides Somiten in Verbindung setzt (Sklerotomkommissur). 

 "Was nun der Oralzone angeht, so entsteht sie dadurch, dass die vordersten Zipfel 

 des urspriinglich schwalbenschwanzformig endenden dorsalen Mesoderms die vor- 

 dere Darmkuppe (vordere Ektodermtasche von Petromyzon nach Kupffer) tiber- 

 und umwachsen, und so einen medianen Zellkomplex bilden. aus dessen hinterer 



