256 GENERAL BIOLOGY 



the mature hydra ; it is rather to a late embryonic form of 

 the hydra, but to one old enough to show the final plan of 

 hydra structure established. 



And so the wonderful process goes forward, yielding by 

 the simplest means results that not the boldest imagination 

 could ever have conjectured*. Each new feature reveals 

 some characteristics of a higher type, and each is soon 

 merged with others still newer. The mesoderm appears, 

 and cleaves apart to form the coelom. Then neural tube 

 and notochord, a heart upon the ventral side and gill slits 

 appear, and it is evidently a vertebrate. But at first it 

 appears to be a vertebrate of very primitive type. It has 

 gills, and a two chambered heart, and a fish -like circulation. 

 Then, more slowly, as we have already seen, these characters 

 are merged into those we call amphibian, and finally it 

 appears in the completed form of a salamander, consonant 

 in general with its species and tribe, and in particular with 

 its immediate ancestors, in form and feature, in proclivities 

 and habits, in faculties and in action. 



What a marvel of potentialities is the egg cell ! What a 

 marvel of performance is the simple cell-mass we call an 

 embryo, that races down the main travelled road of its 

 phylogenetic history, never going astray though countless 

 paths diverge, and only slackens its speed when nearing 

 its proper destination ! 



*"Is it the egg which the hen loves? . . . How should birds 

 know that their eggs contain their young? There is nothing, either 

 in the aspect or in the internal composition of the egg, which 

 could lead even the most daring imagination to conjecture that it 

 was hereafter to turn out from under its shell a living perfect bird. 

 The form of the egg bears not the rudiments of resemblance to 

 that of the bird. Inspecting its contents, we find still less reason, 

 if possible, to look for the result that actually takes place. If we 

 should go so far, as, from the appearance of order and distinction 

 in the disposition of the liquid substances which we noticed in the 

 e gg to guess that it might be designed for the abode and nutri- 

 ment of an animal, (which would be of very bold hypothesis), we 

 would expect a tadpole dabbling in the slime, mucJ? rather than a 

 dry, winged, feathered creature." Paley. 



