AS AN INDEX OF AGE. 33 



" (2) The peculiar manner of interdigitation of the muscular somites as 

 indicated by the sigmoid outline of the mj^ocommata, as seen from their outer 

 faces, and the oblique direction of the membranes separating the muscular 

 cones, has developed a mode of insertion of the myocommata upon the coriura 

 ■which has thrown the integument into rhombic areolae during muscular con- 

 traction. These areolae are in line in three directions, and the folds separating 

 them, particularly at their posterior borders, are inflected in such a manner by 

 muscular tensions, due to the arrangement of muscular cones, as to induce the 

 condition of imbrication so characteristic of the squamation of many fishes." 



The next paper which I must notice is a very important one by Dr. Klaatsch.* 

 While acknowledging my indebtedness to and appreciation of this lengthened 

 paper, I must at the same time agree with Ussow (see p. 202) in regarding 

 some portions of Dr. Klaatsch's work, for example, the section on the 

 " Structure of the teleostean scale from the histogenetic standpoint," as 

 wanting in complete clearness. 



The teleostean scale, its arrangemeiit and 'position in the skin. The scales of 

 Teleosteans are represented by more or less circular plates of hard substance, 

 which exhibit considerable variation in their form. This variation is, however, 

 insignificant in comparison with the general agreement which typical teleostean 

 scales show with one another. Ivlaatsch chooses the cycloid scale as repre- 

 sentative of the ordinary teleostean scale, not only because it presents simple 

 conditions, but because it supplies a suitable object for placing the skin- 

 covering of Teleosteans in line with that of Selachians and Ganoids. As 

 examples of such scales, one may think of such as those of the salmon or of 

 Esox, One distinguishes in such scales two layers : (1) an outer liomorjeneous 

 layer and (2) an irmer fibrillar layer. Each scale is in its anterior half 

 arranged with regard to others in an imbricated fashion, namely, the anterior 

 half of each is covered by three scales, one of which is anterior and dorsal to it, 

 a second anterior and ventral, and a third directly anterior. The centrum of 

 the scale is usually covered over, and scales surround the body in oblique rows. 



For the arrangement of scales in the skin, he gives a figure and description 

 of a transverse section through the skin of a young specimen of Cohitis fossilis. 



Under the epidermis, which contains a large number of mucus cells, the 

 dermis is seen to be raised in a series of projections, each of which corresponds 

 with the posterior free end of a fecale. Each scale lies in an oblique direction 

 from behind forwards, and becomes enclosed in a compartment of the dermis, 

 the so-called "scale pocket." In this scale pocket one distinguishes an outer 

 and an inner wall. The outer wall consists in its posterior part of loose connec- 

 tive tissue containing numerous chromatophores ; in the anterior part the outer 

 wall is composed of tense connective tissue, which is similar to the inner wall 

 of the adjoining anterior scale pocket. 



The fibrous projections of this connective tissue of the outer wall of the scale 

 pocket unite themselves at the anterior border of the scale with the deepest 

 layer of the dermis, in which the fibres have a course parallel to the surface of 

 tlae body. The inner wall of the scale pocket in its posterior part unites with 

 the outer wall of the adjoining posterior pocket. Further forwards it is 



*Klaatsch, 1890. 



NEW SERIES. — VOL. VII. NO. 1. C 



