592 



THE EMBRYOLOGY OF THE FROG 



From the lower ends of each myotome, that is, from those regions 

 lying ventral and close to the mid-line, cells proliferate and move down- 

 ward beneath the notochord as well as upward between the notochord 

 and the myotome. These proliferations form the sclerotomes which are 

 to become the cartilaginous vertebral column. 



Very early, that is, when the tadpole is only about rive millimeters 

 long, the somite proper separates from the lateral plates, and the sclero- 

 tomes separate from the lateral plates. 



Immediately after this separation, there are ventro-lateral out- 

 growths from both myotomes and dermatomes. Those from the myo- 

 tomes become the ventral musculature and extend into the limbs as 

 voluntary muscles, while that from the dermatomes break up into groups 

 of mesenchymal cells, some of which become connected with the inner 

 surface of ectoderm, and form the dorsal covering of the embryo, while 

 some pass between the myotomes to form the connective tissue septa, 

 also called myocommata. (Fig. 423.) 



There are thirteen pairs of somites formed in the trunk of the frog, 

 the two most anterior pairs disappearing in the adult. The full-fledged 

 frog, therefore, has but eleven definite segments. The two anterior 

 somites become part of the occipital region of the head. In the tadpole, 

 there are many more somites than the amount stated above, but with 

 the loss of the tail and the conversion of tadpole into frog, these are lost. 



The following table will not only summarize the history of the 

 somites and spinal nerves, but if carefully studied will show how the 

 adult vertebral musculature which connects the posterior half of one 

 vertebra with the anterior half of the next succeeding one, receives its 

 innervation from more than one spinal nerve. A clear understanding 

 of this will help the student very materially in his comparative anatomy. 

 (See Fig. 336 also.) 



TABLE OF SOMITES, VERTEBRAE, AND RELATED NERVES 



OF THE TADPOLE 



(From Kellicott, after Elliot.) 



