28 FIELD COLUMBIAN MUSEUM ZOOLOGY, VOL. i. 



there nuclei may be distinguished in the deeply staining layer which 

 surrounds the notochord ; but the latter has in many places an appear- 

 ance as if it also had become vacuolated. If so, the mesh work is very 

 fine. I have not in any stage of development seen any such columnar 

 epithelium as that figured by Hasse. (39, Taf. vi., Fig. 14.) 



It may be as well to state here that I have measured the diame- 

 ter of the notochord in specimens up to 44mm. in length, and find 

 that in the vertebral regions, where it is of course most constricted, 

 the diameter has not appreciably increased beyond about .4mm. And 

 yet in a specimen about 12.5011. the opening through the vertebral 

 centrum has a diameter of about .6mm., and this is filled up by the 

 .notochord, which retains the appearance it has in younger specimens. 

 This shows that this organ, instead of being suppressed by the devel- 

 opment of the cartilaginous and bony structures that develop around 

 it, continues to grow slowly until the animal reaches a considerable 

 size. In respect of its notochord, Amia displays a transitional stage 

 between the lower "Ganoids" and the " Teleosts." 



Surrounding the notochord of the specimen 23mm. in length is 

 found the cuticular sheath, whose thickness has increased greatly 

 since the earlier stage. This thickness is now about .O2mm. There 

 .are seen to be indications of the fibrous structure, which has been 

 observed by several writers. Outside of the just described cuticu- 

 lar sheath is the elastica. It forms a thin, highly refractive lin- in 

 cross-sections, and often presents a wavy course. This I attribute 

 to distortion due to postmortem changes. I have never found in it 

 the slightest indication of cells, nuclei, or any other structure. 



The axial structures outside of the notochord and its coverings 

 are of great interest. Throughout the length of the body the lateral 

 halves of the neural arches have met above the neural canal, and 

 their backwardly directed distal ends lie alongside of each other. As 

 is known, they never unite to form neural spines. Interspinous car- 

 tilages are also now developed, both above and below. The proxi- 

 mal member (axonost) is long and slender, its proximal extremity 

 reaching nearly to the tips of the arches, upper and lower. Each 

 is followed distally by two short nodules of cartilage. 



In a cross-section through the dorsal region, the upper arches 

 may be seen to rest on short cartilages which themselves come in con- 

 tact by their lower ends with the elastica (Fig. 9) . Against the right 

 .and left sides of the notochordal rod we find the bases of the lower 

 arches. These, as in the case of the upper arches, are of hyaline 

 cartilage, and their bases are in direct contact with the notochordal 

 sheaths. Distally there is attached to the outstanding process of the 



