British reptiles and batrachians . l division, but nearer the upper than thelower pole, and is followed by longitudinal divisions, after whichthe yolk soon breaks up into numerous smaller cells, as seen inthe above examples. One of the first indications of advance in the animal scale isthe presence of the noto-chord, the forerunner of the vertebralcolumn, or backbone, which is incipiently begun in the egg justso soon as the segmentation process is completed. Then thereappears in the yolk what is called the primitive groove, a delicatelongitudinal line, the sides of which rise up and arch over,
British reptiles and batrachians . l division, but nearer the upper than thelower pole, and is followed by longitudinal divisions, after whichthe yolk soon breaks up into numerous smaller cells, as seen inthe above examples. One of the first indications of advance in the animal scale isthe presence of the noto-chord, the forerunner of the vertebralcolumn, or backbone, which is incipiently begun in the egg justso soon as the segmentation process is completed. Then thereappears in the yolk what is called the primitive groove, a delicatelongitudinal line, the sides of which rise up and arch over, forminga canal, the lining of which becomes the spinal chord; while be-neath it a gelatinous rod develops into a noto-chord {chorda dorsalis).Hence ultimately the backbone. Now in the Tunicates, as Clausexplains to us, there is an embryonicdevelopment, which up to a certainpoint presents a great resemblanceto that of the lower vertebrates. Flg ^-TBntate TadP°le- First, the egg segmentation, subsequently a structure like a chorda. 46 REPTILES AND BATRACHIANS. dorsalis, a neural tube, a heart, respiratory organs, and so forth,analogous to those of Vertebrates, until quite a respectablelittle tadpole is developed (fig. 17, last page). In the frogs egg something of this kind may be watched fromday to day. The germ begins in the upper part. The illustrations—much magnified, especially the earlier ones, as given in fig. 18—enable us to trace the still further progress. I select them onaccount of their distinctness, though tadpoles of a French, not anEnglish frog, but with a similar development. In the earliest (a)we see that the embryo assumes a form before it leaves the egg,but there is no mouth, and no sign of an eye. Between iVand .S isa very slight depression, which is where the future mouth will be,as you will observe by comparing the same spot with that infig. b and fig. c. In the two latter the tail is large and strong,compressed laterally for swimming; no limbs appear a
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