ii6 THE BIOLOGY OF THE FROG CHAP. ^ 



somite, collect around the notochord, forming a tubular in- 

 vestment. From the dorsal side of this mass ridges or folds 

 grow up and surround the spinal cord. The mesoderm cov- 

 ering the notochord then becomes divided by transverse septa 

 which alternate with those between the somites, but these do 

 not cut across the notochord itself. The segments they cut 

 off represent the vertebrae ; they soon become cartilaginous, 

 and finally ossify. The cartilaginous sheath grows inward 

 at the ends of the vertebrae, constricting and finally cutting 

 through the notochord, so that in the adult all that remains 

 of this structure are small portions inclosed within the centra 

 of the vertebrae. 



Metamorphosis.- -At the time of hatching the tadpole is 

 a fish-like creature, having a long, vertically flattened tail, by 

 means of which it swims through the water. The sides 

 of the tail show the markings of the muscle segments 

 through the skin. The flattened expansions of the integu- 

 ment on the upper and lower sides of the tail are thin and 

 nearly transparent, so that one may easily observe with a 

 microscope the blood flowing in the capillaries. 



The mouth breaks through into the archenteron a few 

 days after hatching, the larva, previous to this time, living 

 at the expense of the food yolk in the alimentary canal. 

 The intestine increases very rapidly in length, and becomes 

 coiled iu the form of a spiral, which may often be seen 

 through the ventral body wall. The external gills grow 

 rapidly after the tadpole is hatched, and soon are converted 

 into long, branching tufts. Three pairs of external gills are 

 developed, the posterior pair making its appearance after 

 the first two. The gill slits grow about the time the mouth 

 is fully formed, and the water which is taken in at the mouth 

 is passed through the gill slits to the exterior. In addition to 

 the external gills there are developed somewhat later four 





