Developmental history of primaiy segments of the vertebrate head. o97 



the foregut and occupy a position ventrad to the segmented mesoderm. 

 As to the ontogenesis of these pockets that Hatschek regards as 

 rudimentary, he says (in: Verh. anat. Ges., 1892, p. 144): "Sym- 

 metrical in their origin they become in later development unsym- 

 metrical, the right one is extended and the epithelial cells of the wall 

 become quite flat and enclose a large triangular space, ventral to the 

 chorda, in the anterior end of the body. The left sac has a thick 

 wall, develops cilia and takes a diagonal position under the chorda, 

 and finally forms a little opening on the left side after dividing into 

 two divisions. The division on the left side which has the external 

 opening is wider and ciliated, the one to the right forms a blind 

 pocket and is narrower. The external opening which enlarges is 

 ventral and to the left of the chorda" (Author's translation). 



Sewertzoff 1), '95, is unable to find pro-otic segments in embryos 

 of Acipenser and Siredon. After a careful review of the literature he 

 concludes that "between the auditory vesicle and the anterior dorsal 

 arch of the occipital region three somites are found in the shark 

 (the 5th, 6th and 7th somite); but only two in Amphibia and Birds. 

 In Amphibians (Anura) and Birds, one more mesodermic segment is 

 found in the pro-otic region. In this manner the conclusion to which 

 we come is as follows : there was a time when the cephalon was seg- 

 mented in the ancestors of the present vertebrates. . . , The somites 

 that lie in front of the 2nd somite of the meta-otic region of the 

 Urodeles [which corresponds to the 7th head somite of the shark 

 (Van Wijhe, Hoffmann) and the 6th of the lamprey (Hatschek)], 

 are segments common to all Craniota and consequently primitive head 

 metameres. Gradually the following segments which originally be- 

 longed to the trunk were added to the component elements of the 

 head ; at first the occipital arch and the myotome which lies before 

 it, the Urodeles have remained at this stage which is common to all 

 Gnathostomata ; in Anura another myotome is added to the head; 

 finally we have in the case of Fishes, Sauropsida and Mammalia the 

 most complete expression of this process, a whole series of trunk 

 segments being added to the component elements of the head. In 

 general the evolution in the back part of the head can be traced to 

 the disappearance of metameres which took place from in front back- 

 wards and to the gradual transformation of the front segments of the 

 trunk into the back part of the head" (Author's translation). 



1) in: Bull. Soc. imp. Nat. Moscou, No. 2, p. 273. 



