80 EMBRYOLOGY OF THE LOWER VERTEBEATES ch. 



of many Anura. In this case they appear very precociously in 

 development, being indeed in some cases the first definite organs 

 to become visible on the surface of the embryo. Fig. 48 illustrates 

 the development of the cement-organ in the common toad {Bufo 

 vulgaris) from the time of its first appearance up to the time of its 

 atrophy. The organ is seen to he at first crescentic as in Lepidosiren 

 then to become V-shaped and finally to become paired by the atrophy 

 of its median portion. 



When at the height of its development, the cement-organ shows 

 characteristic differences in form and position in different species 

 of Anura and is consequently of use in identifying the species of 

 Tadpoles. 



The general appearance of the Anuran cement-organ as observed 

 in sections is illustrated by Fig. 49. The glandular layer is com- 

 monly said to belong 

 to the superficial layer 

 of the ectoderm but 

 this does not seem by 

 any means certain. 



Pigment of the 

 Skin. — One of the con- 



Fig. 49.— Section through the cement-organ of a Frog spicuOUS features of the 

 Tadpole (Ra-iui temporaria) 8 mm. in length. (From g ^ n [ n fcjjg maioritv of 

 Assheton, 1896.) -, r , , , . J , , % , 



Vertebrates is the fact 



c.o, cement-organ; ecfi, superficial layer of ectoderm; fj, a ti* ia Pnlnnr-Arl Vw 



«#, deep layer of ectoderm. mal ll 1S coioure a Dv 



the deposition within 

 it of excretory matter in the form of pigments. This is of physio- 

 logical importance to the organism in two different ways, firstly in 

 that it gives to the particular species its characteristic coloration, 

 and secondly it serves to protect the underlying living tissues from 

 the harmf ul influence of light rays. 



A certain amount of pigment may be formed within the proto- 

 plasm of the ectoderm cells. For example in frog tadpoles of about 

 an inch in length, numerous fine granules of melanin are crowded 

 together near the surface of the outer layer of ectoderm cells, just 

 beneath the cuticular superficial layer. 



But by far the most important part of the pigmentary system of 

 Vertebrates consists of mesenchyme cells with pigment-laden cyto- 

 plasm which are positively helio tropic during life and creeping by 

 the extrusion of slender pseudopodia, like those of Foraminifera, 

 crowd together immediately beneath the ectoderm and form there 

 a light-proof layer, some of them even wandering into the substance 

 of the ectoderm between its constituent cells. 1 



The chromatophores, during the process of development, commonly 

 become specialized in different directions so that in the fully developed 



1 The interpretation of the branched chromatophores as mesenchymatous in origin 

 appears to the author to accord best with observation but it should be mentioned that 

 some regard them as modified ectoderm cells, as for instance Winkler (1910). 



