MUSEUM OF COMPAUATIVK ZOOLOGY. 125 



formed, the scales bear upon their outer surface numerous slender coni- 

 cal spines or teeth. These were first described by Rcissner ('59, p. 2G0). 

 They stand up from the surface not quite perpendicularly, but have a 

 slight inclination toward the tail (Plate I. Fig. 7, and Plate IV. Figs. 2G, 

 28). They are transient structures, which frequently leave no trace 

 of tiieir existence in the adult scale. They begin to be formed when the 

 scale is still comparatively thin (75-100 /x), and like it they owe their 

 origin to the cells of the outer or scleroblastic layer of the dermis from 

 which they are principally formed. 



The first step in the formation of a spine is an increase in the number 

 of the cells lying upon the upper surface of the scale just under the 

 basement membrane. A thickening is thus formed which rises up 

 against the base of the epidermis in the form of a papilla (Plate I. Fig. 5). 

 The basal layer of epidermal cells becomes arched over it, and the co- 

 lumnar character of the cells much more pronounced. The papilla 

 increases in height rapidly, pushing forward into the epidermis, with its 

 point directed slightly caudad, but remaining at its base little if any 

 larger than at first. The cells of which it is composed are the same in 

 character as those overlying the scale ; in the axis of the papilla they 

 are crowded together without any definite arrangement, but at its periph- 

 ery, whei'e they adjoin the basement membrane which separates them 

 from the epidermis, there is perceptible a certain degree of regularity in 

 the arrangement of the nuclei ; they lie rather closely appressed, being 

 elongated radially to the axis of the cone. 



After the papilla has attained a considerable height there appears 

 upon its tip a thin crust of calcareous matter secreted by its cells ; this 

 gradually extends down over the sides, forming a conical cap (Plate II. 

 Fig. 10), which is pushed forward by growth from below as the papilla 

 increases in height. In the early stages of its formation, the papilla 

 merely indents the lower surface of the epidermis, but as the point of- 

 the spine is pushed forward it causes a swelling upon the outer surface 

 of that layer (Plate I. Figs. 5, 6). At the same time that this is taking 

 place the thickness of the spine cap is being increased by the addition 

 of new material from within. By the continuation of these processes 

 the point of the spine is made to pierce the epidermis and so lie free to 

 the exterior, while the basal end joins the scale already formed below. 

 There is no joint or hinge of any kind at the place where the two 

 unite, but the material of the spine is directly continuous with that of 

 the outer part of the scale. The cells of the papilla are thus entirely 

 shut off from the other scleroblastic cells which lie over the outer sur- 



