Boo mee Me oe ont a eR imal ia. |” E cro 
3 d yrs 
1894.] The Origin of the Vertebrate Skeleton. 639 
ing of ectoderm cells into the region between the basal epithe- 
lium and the corium. From these cells are produced at first 
extremely minute horny rods, and these, later, together with 
their parent cells, sink through the corium into the position 
they finally oceupy, where no one, not tracing their history in 
detail, would suspect their ectodermal origin. Even in Tor- 
pedo, where no horny rays occur in the paired fins of the adult 
ingrowths of ectoderm into the axial portions of the fin exist. 
Atthis point one author supports Rabl in his view that the 
unpaired fins are not derived from the fusion of paired rudi- 
ments. The opposite view is fastened upon Dohrn, regardless 
of the fact that it was first shown to be probable by J. K. 
Thacher and later supported by Balfour. Dohrn’s special 
contention was that the fins, paired and unpaired, were deriva- 
tives of the parapodia of the worms, and later, Paul Mayer 
claimed to have found structures—“ parapodoids"— which rep- 
represented these. These “parapodoids” are, according to 
Klaatsch's view, the early placoid organs. 
In studying the development of true bone, Klaatsch studied 
Salmo salar. Here the earliest to appear were the opercular 
bones, and but little later those of the shoulder girdle and 
those arising in connection with the teeth, later those of the 
cranium. The details of the formation of the scleroblasts for a 
few of these bones is given, including the squamosal, opercu- 
lum, clavicula, dentary, and the osseous fin-rays. In the case 
of each, the osteoblasts are derivatives of the ectoderm. "The 
squamosal is especially interesting, since it begins from out- 
growths at the point of the infolding of the mucous canals, and 
is developed in connection with these organs. At first it is 
connected solely with them, and is plainly a membrane bone ; 
later it comes into contact with the otic capsule. Klaatsch | 
sides with those who would make no sharp distinction between 
cartilage- and membrane-bones, and regards not only the 
squamosal but the cranial roof and the ossifications which ap- 
pear in the cranial roof and on the primordial cranium as 
having their origin in bones developed, like the squamosal, for 
protection of the cutaneous sense-organs. 
