44 REPTILES AND BATRACHIANS. 



found in the larval Ascidians, or Tunicates. But unfortunately 

 the Tunicates are such very degenerate vertebrates that very little 

 can be got from them for the elucidation of the problem." 



Of that " little " they can be made to suit our purpose by com- 

 paring the metamorphosis which they undergo with that of the 

 frog, and thus afford us an example of that " higher " and " lower " 

 development, and also some idea of the part which an embryo 

 plays in this. 



Both they and the frog begin life — that is, a free, independent 

 life — as a tadpole. In the egg they really do begin life, and we 

 will begin our examination of frogs' eggs from the time they are 

 laid. This is in the spring, when you may see a jelly-like mass 

 down among the weeds in a pond, and which on examination you 



Fig. 15. — Frogs' eggs and early tadpoles : natural size. 



find to be composed of numerous globular bodies all clinging 

 together, and in the centre of each a small, dark, round body, which 

 is the yolkof the egg ; while the glutinous envelope is the " white " 

 of the egg. They are laid in a long string, but owing to the glairy 

 envelope which soon swells in the water to about a quarter of an 

 inch in. diameter, the whole takes the form of an irregular trans- 

 parent clump full of dark specks. It is the latter which, after a 

 few hours, undergo a rapid change that can be observed under 

 the microscope. You will see each one ot them dividing up into 

 a great number of sections or cells, as if acted upon by some 

 chemical agent. This is called segmentation, and is what every 

 egg — whether fish, reptile, or fowl, the ovum of every vertebrate 

 animal, in fact— undergoes. This cleavage, or yolk subdivision, 

 however, does not always present the same appearance in the eggs 

 of various creatures, though always it is the first stage of hatching. 

 The segmentation may be " equal " or " unequal." Sometimes it 

 proceeds more rapidly in one portion of the egg than another. 

 But, as a rule, the cleavage duplicates itself; first in halves, then 

 quarters, then 8, 16, 32, etc., sections, producing at last a granu- 



