718 ALLTS. [Vor cat 
different parts of the vertebral column and of the body. Where 
it first begins, at what age, and the amount of it at different 
ages, I have not attempted to determine, as it belongs more 
properly to a later study. It certainly begins at very early 
ages, for even in 12 mm. larvae it is already plainly evident. 
The course alone of the septa, in part vertebral and in part 
intervertebral, sufficiently indicates this, for it it generally ac- 
cepted that the septa are, primarily, through their entire length, 
strictly vertebral in position. As the dorsal arches arise in the 
septa, even these arches must, in 12 mm. larvae, have shifted 
somewhat from their original position. In this shifting, the 
arch and septum would naturally carry with them the nerve 
that innervates the muscle segment next posterior to them. 
Such at least is the case in Amia ; for in larvae, as in the adult, 
the nerve that issues behind an arch innervates the muscle 
segment that lies behind the septum of the vertebra next 
anterior to that arch. My work thus tends to confirm Koll- 
mann’s conclusion (quoted No. 24, p. 20) that the dorsal arches 
in human embryos shift backward relatively to their vertebrae. 
Schmidt, from an anatomical and histological investigation 
of the vertebral column of the adult Amia, concludes (No. 110, 
pp. 751 and 755) that there were, originally, in each body seg- 
ment, two vertebral “Anlagen.” These “Anlagen”’ in the 
trunk fuse to form a single complete vertebra, the dorsal arch of 
the posterior half-vertebra, or intercentrum, remaining as the 
dorsal arch of the complete one, and that of the anterior half- 
vertebra, the centrum, persisting as a cartilaginous process, 
on the dorsal surface of the vertebra, immediately in front 
of that arch. This process, or rudiment of a dorsal arch, 
as he considers it, can be nothing more nor less than the 
anterior half of what I have described as the dorsal cartila- 
ginous process of the vertebra, a part of that vertebra and not 
of an arch. In larvae, as in the adult, there is not the slightest 
indication of an arch that has disappeared, or of two vertebral 
bodies that have fused. Schmidt’s conclusion, which simply 
confirms Ihering’s earlier statement (No. 61), thus finds no 
confirmation in my work, so far as it has gone. Regarding 
Gadow and Abbott’s conclusions (No. 39, p. 298), based on 

