THE naturalist's OCCUPATION. 4I 



this structure extends through the greater part of the 

 head region of the embryo, from which we may safely 

 infer that, at least, so much of the primordial cranium 

 as possesses this structure must be regarded as a direct 

 continuation of the vertebral axis, even though no 

 distinct outlines of segments appear in it. In the ab- 

 sence of such outlines, the inquiry turns upon indica- 

 tions which may betray their former existence. For 

 evidence of this sort, the investigator continues his 

 search, first of all, in the posterior region of the head, 

 since this is demonstrably the least modified. Now it 

 is just here that embryology has been able to demon- 

 strate, in some of the lower fishes, the existence of at 

 least one genuine vertebral segment. In a remarkable 

 shark from Japanese waters, which Mr. Garman of 

 Harvard has recently baptized with the name, Chlaniy- 

 doselachns aiigidnens, as I learn from Dr. Ayers, who 

 has lately studied its cranium, there are unmistakable 

 evidences of from three to five cranial vertebrae. Indi- 

 cations of a considerable number of primordial seg- 

 ments, or protovertebrae as they are called, have been 

 discovered in the hind head of the embryo of the 

 amphibian, the reptile, the bird, and even the mammal. 

 In the lowest representative of the fishes of to-day, 

 the much-talked-of Amphioxus, the segments and the 

 chordal axis extend from end to end ; and the head 

 merges so completely in the trunk, that the most search- 

 ing examination has scarcely yet been able to fix any 

 boundary line. Although Amphioxus takes an isolated 

 position, and may have sacrificed some elements of 

 its head in exchange for the material enjoyments of a 

 semi-vegetative existence, still it must be admitted as 



