42 THE TERIODIC GROWTH OF SCALES IX GADID.E 



ectoderm, and not from the connective-tissue layers. Those beneath tlie scale 

 (the lower scale layer) are budded off from the ectodermal invagination, 

 which grows in under the posterior end of the scale. 



To Ussow we are indebted for a paper on the development of the cycloid 

 scale of Teleosteans.* This author differs in a number of points from Klaatsch's 

 views on the same subject. I shall endeavour to give a summary of Ussow's 

 paper. 



The scale of Teleosteans is built up of two layers, the structure of which is 

 not agreed upon by the various authorities. Hofer, for instance, considers the 

 first and otdermost layer of the scale to be composed of a form of dentine 

 which he terms ''h//alode7iti7ie," and speaks of the transformation of this 

 special tissue into common dentine. The second and innermost layer of the 

 scale is, according to Hofer, formed from the dermis. 



Klaatsch, from the presence of minute osseus bodies in the first layer of 

 the scale, holds that this layer is built of ordinary bony tissue. He believes 

 that the second layer of the scale is formed out of connective tissue which is 

 developed from the scale pocket. 



Leydig and Baudelot regarded the scales of Teleosteans simply as a con- 

 glomeration of calcareous concretions or little scale bodies. 



In regard to literature dealing with the development of scales, Ussow cites 

 the following three papers : — 



(1) Klaatsch, "Zur Morphologic dcr Fischschuppen and zur Geschichte der 

 Hartsubstanzgewebe." 



(2) Hofer, " Ueber den Bau und die Entwicklung der Cycloid und Ctenoiil 

 schuppen." Berl. Gesellschaft filr Morphologie zuul Physiologie in Milnchen. 

 1889. 



(3) Maria Sacchi, " Sulla struttura del toguniento negli embryoni ct 

 avanotti del Salmo lacustris." Red. del Inst. Lomhardo, vol. xx. ]\lilano. 

 1887. 



The species which Ussow selected for the study of scale development were 

 the following : — In the family Cobitidee, Cohitis iicnia, Cohitis barbaiida, 

 Cobiiis fossilis ; in the family Cyprinidse, Leucasjnus dclineatus, Leuciscus 

 rutilus, Carassius vulgaris. 



As the origin and development of teleostan scales takes place from the 

 mesoderm elements of the dermis, Ussow first gives some notes on the 

 epidermis and dermis of Cohitis tivnia as an example. In young examples 

 of Cohitis ttenia the skin is imperfectly developed and the epidermis is 

 thicker than the dermis. In such young stages (embryos 4 cm. long) mucus 

 cells are present in the epidermis in great number. The dermis consists at 

 this stage of numerous fibres and cells embedded in an intermediate ground 

 substance. In older forms tlie epidermis becomes much thicker and the 

 nuicus cells increase in number. The dermis also becomes thicker at the 

 expense of connective-tissue fibres Avhich cross under one another almost at 

 right angles ; these fibres, surrounding the body of the animal, lay themselves 

 down in diagonal lines in relation to its longitudinal axis. The dermis is 

 separated from the muscles by an epithelial layer of cells, not clearly marked 



. • Ussow, 1897. 



