INTRODUCTION 



559 



UNICELLULAR GLAND 



Fig. 264. Diagrams pertaining to the skin of bony fishes. (A and B after H. V. Wilson: 

 Bull. U. S. Fish Commission, Vol. 9, 1889, reprint, 1891; C after Kingsley: Comp. 

 Anat. of Vertebrates, 1912, P. Blakiston's Son & Co., Phila.; F from Reed; Am. Nat., 

 41.) (A) Section of ectoderm (primitive epidermis) of 39-hr. embryo of Serranus 

 atrariiis. the sea bass. (B) Epidermis of sea-bass embryo of 59 hrs. (C) Skin of the 

 lungfish, Protopterus. (D) Integument of teleost fish with special reference to scales. 

 (E) Higher power of epidermal and dermal tissue overlying scale in D. (F) Poison 

 gland along pectoral spine of Schilheodes gyriniis. 



primitive epidermal layer of the skin eventually is composed of two simple 

 cellular layers, an out(.r protective periderm, and a lower, actively proliferat- 

 ing stratum germinativum. It is to be observed further that the periderm in 

 the recently hatched frog embryo possesses ciliated cells (fig. 267H, I). These 

 cilia, as in Amphioxiis (fig. 249B), are used for locomotor purposes, and also 

 function to bathe the surface with fresh currents of water. As such, they 

 probably play a part in external respiration. 



The periderm forms a protective covering for the actively dividing and dif- 

 ferentiating cells below. In the mammals, the periderm occasionally is called 

 the epitrichium, as it eventually comes to rest upon the developing hair. In 

 Amphioxm, there is no periderm, and the epidermal tube (epidermis) remains 

 as a single layer of one cell in thickness (fig. 250E, F). 



2) Origin of the Dermal or Mesenchymal Component. In Amphioxus, the 

 thin lateral and ventro-lateral walls of the myotome give origin to the derma- 

 tome which comes to lie beneath the epidermal wall. From the dermatome 

 arises the dermis or connective-tissue layer of the skin (fig. 250E, F). The 



