70 EMBRYOLOGY OF THE LOWER VERTEBEATES oh. 



while from its deeper layers arise the main portions of the central 

 nervous system. 



THE EPIDERMIS 



The ectoderm covering the surface of the embryo becomes 

 converted, normally, into the epidermis of the fully developed 

 individual. Very usually the embryonic ectoderm consists of two 

 layers of cells, the lower layer composed of actively living cells, the 

 superficial of flattened plate-like protective cells. This outer layer 

 has been termed by Krause the periderm : its superficial protoplasm 

 is commonly hardened to form a cuticle in the strict sense of 

 the term. Normally it plays no active part in development and is 

 shed at an early period. 



The deep layer of the ectoderm on the other hand is active. 

 Its cells multiply so that it becomes several layers thick : the outer 

 layers become cornified to form the horny stratum of the epidermis 

 while the deeper cells, composed of active living protoplasm, form 

 the stratum of Malpighi. 



The outer layer of ectoderm cells may be for a time ciliated. 

 This is well seen in young Amphibian embryos (Assheton, 1896). 

 In Bana temporaria the 6 -mm. embryo possesses ciliated cells 

 scattered thickly over its surface, the movement of the cilia being 

 such as to drive a current of water tailwards over the surface of the 

 embryo. When the external gills develop, a specially strong ciliary 

 current sweeps backwards over them and it is noteworthy that this 

 current passes over the olfactory organ en route to the external 

 gills so that the olfactory organ possibly plays an important part 

 in testing the quality of the water going to the respiratory organs. 

 The ciliary apparatus is sufficiently powerful at the stage in 

 question to cause an embryo of this stage when laid on the bottom 

 of a flat glass vessel to slide along at the rate of a millimetre in 

 from four to seven seconds. As development proceeds the ciliation 

 becomes less and less prominent and in a 20-mm. tadpole it has 

 almost disappeared except on the surface of the tail which remains 

 richly ciliated until the time of metamorphosis. This persistence 

 of the tail cilia is doubtless correlated with the fact that the skin 

 of the tail plays an important part in the process of respiration. 



HORNY DEVELOPMENTS OF THE EPIDERMIS 



Scales. — In many terrestrial Vertebrates the horny layer of the 

 epidermis becomes so thickened and hardened as to become practically 

 rigid. In such cases the flexibility of the skin as a whole is retained 

 by the thickened areas of epidermis being separated from one 

 another by lines along which thickening does not take place. The 

 thickened portions now form epidermal scales of the type seen in 

 Eeptiles. They may take the form of simple rounded projecting 

 bosses or tubercles as in Chameleons, or they may be flattened 



