DEVELOPMENT OF THE FROG S EGG. 



301 



A still later stage of development demonstrating a more 

 marked differentiation of the tail from the head region is shown 

 by Fig. 4, while the tadpole in Fig. 5 represents the external 

 features of these tadpoles during the free-swimming period. In 

 this latter picture, the sucker, the eyes, the external gills and the 



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FIG. 4. 



tail fins are readily made out. The sharpness of the fold in the 

 trunk in both instances indicates a restricted localization of the 

 alteration produced in the protoplasm of the developing ovum. 

 These creatures are unable to straighten out the flexure of 

 the trunk. They swim, as was noted before, by means of 

 ordinary tail movements, the vigor of which is not impaired 



FIG. 5. 



in any single instance. As can readily be understood by refer- 

 ence to the figure, however, the abnormal position of the caudal 

 long-axis, with regard to the long-axis of the head and remaining 

 portion of the trunk, is such that the swimming movements of 

 the tail tend to force the embryo about in a circular or spiral 



