Ri i a «i 
1894.] The Origin of the Vertebrate Skeleton. 637 
the statements of his preliminary, and showing that ectoder- 
mal ingrowths occur in the birds in just such positions as to 
justify the view that they gave rise to skeletal structures. 
Some of these, according to Goronowitsch, found their destiny 
in the cutis, a fact to be remembered while considering the 
work of Klaatsch, outlined below. A little later (93*) Gorono- 
witsch published a short note in which, among other points, he 
claimed that Miss Platt had not made good her thesis that 
these mesectoderm cells gave rise to the cartilage. Miss Platt’s 
final paper will, we understand, soon appear. 
The most important and most detailed paper of all is that of 
Klaatsch, which appeared in April of this year. Its title—“ On 
the Origin of the Scleroblasts. A Contribution to the Know- 
ledge of Osteogenesis "—shows its scope. Wecan give but the 
merest outline of the points detailed in the 90 pages of the 
paper. 
The first point considered is the development of the placoid 
scale. 'This, as is well-known, consists of two portions, a 
harder outer portion, the enamel secreted by the basal ends of 
cells of undoubted ectodermal origin; and a deeper dentine 
which, up to now, has been universally regarded as of true 
mesodermal nature. Klaatch studied the development of the 
placoid organ in several species of Acanthias, Mustelus and 
Heptanehus. These presented various differences, but in 
general, they agreed in the following features. In the earlier 
stages the ectoderm is two cells in thickness, a flattened super- 
ficial layer and a deeper cubical or columnar layer. Between 
this last and the corium is a clear space, and there is no con- 
tinuous basal membrane. A little later this deeper layer be- 
gins to undergo modifications, cells being budded from it into 
the clear space. These cells are readily seen to belong to the 
ectoderm, not only from the directions of the mitotic spindles, 
but from the fact that their nuclei are greatly larger than 
those of the corium, the only other layers from which they 
could arise. These cells are the scleroblasts. They are not 
scattered irregularly through the clear space, but are more 
abundant in some places than in others thus early marking 
out the positions of the later placoid organs. With the modi- __ 
