136 BULLETIN OF THE 



ill the scleroblastic dermal cells to become osteoblasts, and m couuection 

 with this process it has developed a system of tubules for supplying 

 them with nourishment. Moreover, instead of being formed simply as a 

 continuation of the process by which the spine is produced, it has come 

 to develop independently of the sjjine, for it is only in a late stage of its 

 growth that the two become united. Thus the Ganoid scale plate seems 

 to have arisen from the placoid basal phxte by increase m size and witli 

 important modifications. 



2. The spine, on the other hand, has become reduced in size and in 

 complexity of structure, and is in Ganoids (Lepidosteus) only. a rudi- 

 mentary organ arising late and disappearing early, as is frequently the 

 case with degenerate structures, the " wisdom teeth " of man being a 

 familiar illustration of this. 



Hertwig's view is that the scales of Lepidosteus have arisen by the 

 fusion of numerous smaller basal plates of scales of the Selachian (pla- 

 coid) type. Each spine upon a scale of Lepidosteus therefore represents 

 a primitive placoid scale, and the whole Ganoid basal plate has arisen by 

 the fusion of as many simple scales as the total number of spines formed 

 upon its surface. Ivlaatsch objects to this interpretation, since the 

 number of spines is so large and wholly indefinite, and because the 

 spines lack such an orderly arrangement as that which the scales have 

 in selachians. 



My own view in regard to this matter is essentially the same as that 

 expressed by the latter author. Though the Ganoid scale must be 

 regarded as a moi'e highly developed basal plate than that found in the 

 Selachians, its origin is not due to the fusion of many small ones, but 

 rather to the calcification which in Selachians originated in connection 

 with the formation of placoid spines, having become in Lepidosteus 

 an independent process no longer dependent v;pon the impulse given by 

 the growth of the spine. The hereditary tendency toward the growth 

 and calcification of papillae still shows itself, however, in tlie formation 

 of tlie small spines, though these are retarded in time and but feebly 

 developed. 



As long as each spine had a basal plate, as in Selachians, the spatial 

 requirement of this plate exercised a controlling influence upon the num- 

 ber and the arrangement of the scales (^= spines). When now, as in 

 Lepidosteus, the spines have come to arise independently of the under- 

 lying plate, such restraint is removed, and we consequently find an in- 

 crease in the number of the spines and a lack of regularity in their 

 arrangement. 



