26 FIELD COLUMBIAN MUSEUM ZOOLOGY, VOL. i. 



condensed than the tissues which lie between the arches. On the 

 lower side of the notochord there may be seen at intervals, in sagittal 

 sections of the more advanced lomm. specimen, little masses of more 

 dense, more deeply staining tissues, the beginning of the lower 

 arches. In cross sections these may be traced from the lower outer 

 side of the notochord downward to near the aorta, their lower ends 

 staining a little more deeply in haematoxylin. In the less advanced 

 specimens the indications of the arches, upper and lower, fade out 

 posteriorly ; but in other specimens they may be followed into the 

 tail, where also the cartilage becomes better differentiated. Here 

 the lower arches may be seen to extend down the sides of the aorta 

 and pass into the indifferent tissue which surrounds the caudal vein. 

 The upper arches, however, have become very short, mere cartilagi- 

 nous papillae at the sides of the spinal marrow. 



When we come to examine specimens having a length of i5mm. 

 we are able to observe a number of important changes in the axial 

 structures. The notochord has attained a diameter of about .3mm. 

 There is present a distinct cuticular sheath, the thickness of which I 

 make .0033111111. Outside of this is an extremely thin, but distinct, 

 membrana elastica. Lying within the cuticular sheath, and surround- 

 ing the notochord there is present a very distinct epithelial layer. 

 The nuclei of the cells stain deeply, and the whole layer has a thick- 

 ness of about .oo66mm. How this layer of cells originates I am tin- 

 able to say. I find no traces of it in a specimen i2mm. in length, 

 except it be the already described cell-less layer just inside the cuti- 

 cular sheath. Scheele describes and figures a distinct epithelial 

 layer as existing in the Trout (55). Goette has denied the exist- 

 ence of such a structure, but it has been demonstrated by a number 

 of competent observers, as Gegenbaur, Grassi, etc. 



In the anterior dorsal region the upper arches rise two-thirds the 

 height of the spinal cord, passing above into the delicate connective 

 tissue which closes in the latter organ. In the posterior region they 

 are very short. When sagittal sections of this length are examined, 

 we find that in the anterior region some new elements are beginning 

 to show themselves. These are small masses of incipient cartilage, 

 which are placed immediately in front of the bases of the upper 

 arches (Fig. 8). Whether or not these masses are ever at any 

 time united with the bases of the arches by continuous cartilage, 

 incipient or well developed, I find myself unable to say definitely. In 

 nearly all cases there may be seen at least a narrow line of connective 

 tissue cells surrounding each little mass and cutting it off from the 

 bases immediately in front and behind. But occasionally, both while 



