OF THE HEAD IN THE BATRACHIA URODELA. 103 



Further, if the Amphibian head includes fewer segments, it may 

 be aslved whether this condition is primary, or due to the return 

 of segments to the trunk which formerly held a place in the head. 



It is essential before attempting to answer these questions to 

 determine how many head-segments can actually be tx-aced in 

 ontogeny. Other observers have attacked the problem, but 

 their results are not in agreement. With a view to settling this 

 point I undertook the study of the development of the head- 

 region in the Axolotl [Ambly stoma tigrinuTn). 



My method has been to reconstruct series of sections on 

 paper. For this purpose it is important to have a very com- 

 plete set of stages cut in various directions. I have to thank 

 Dr. J. W. Jenkinson for the loan of a lai-ge number of excellent 

 series of sections filling up the gaps in my own preparations. An 

 appropriate number of stages has been selected for represen- 

 tation in the figures given in the text ; but it will be undei-stood 

 that intermediate stages have been examined. Of these figures, 

 nos. 33-38, 40-43, and 45 49 were drawn from Mr. Jenkinson's 

 series. 



Description. 



Before describing my own observations it will be well to give 

 a brief account of the results obtained by previous workers. A 

 good general description of the development of the skull of the 

 Axolotl has been given by Parker (7). He describes the basilar 

 plate, or floor of the cranium behind the pituitary fossa, as formed 

 of a parachordal extension of the trabecular bars, combined with 

 " proper occipital parachordals behind." The latter give rise to 

 the occipital condyles ; but their exact relation to the myotomes 

 is not elucidated, although Parker notices that the glossopharyn- 

 geal ganglion lies behind the auditory capsule between the first 

 and second myotomes. Stohr (11) has given a very similar account 

 of the development of the skull of Siredon [Ambly stoma). Like 

 Parker he found that the postpituitary region of the skull develops 

 from three separate sources : the anterior parachordals (" Balken- 

 platten ") derived from the trabeculae, the auditory capsules, and 

 the occipital arches. By means of reconstructions of sections he 

 made out clearly the origin of the posterior occipital arch. These 

 arches, compared to vertebral arches, grow over the brain above, 

 and along the sides of the notochord (occipital parachordals of 

 Parker) join the backward extensions of the trabeculse (" Balken- 

 platten "), and enveloping the notochord form the basilar plate, 

 which subsequently fuses with the auditory capsules. Stohr, 

 however, did not make out the exact position of the occipital 

 element with regard to the nerves and myotomes. 



Sewertzoff (9) was the first author to attack this problem directly. 

 According to his account there are two metaotic somites, giving 

 rise to two myotomes in the embryo (text-fig. 50, B, p. 116). 

 The 1st disappears later. The 2nd, corresponding to the vagus, 

 remains. Behind it, in the septum between the 2nd and 

 3rd, develops the occipital arch. The first trunk-segment 



