. Text-book of embryology. Embryology. 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 deve
. Text-book of embryology. Embryology. 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 , cement-organ; ecfi, superficial layer of ectoderm; fj,ati* ia Pnlnnr-Arl Vw «#, deep layer of ectoderm. mal ll 1S coiourea 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 ben
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