COMPENDIUM. 287 



Simple prolongation of the extremities may next produce a vermicu- 

 lar shape. 



Thence the early appearance of embryonic animals may resemble 

 worms. 



These are hypotheses, indeed, to be sustained or overthrown by obser- 

 vation. But, in certain respects, they are undoubtedly not altogether un- 

 founded. 



I have already remarked the opinion of some learned authors on the 

 irritability sometimes betrayed by ova, and if I rightly understand them, 

 that the ova are endowed with spontaneous motion. I must be permitted 

 to repeat, that this is only inaccuracy of expression, for it does not appear 

 that any such faculty belongs to the ovum itself: — otherwise the power 

 must reside in the integument, whether a calcareous shell or a membra- 

 naceous capsule, which is not a logical inference. Any irritability or mo- 

 tion should be, therefore, ascribed to the contents, as having attained a 

 particular stage. But motion, that is spontaneous motion, belongs more 

 properly to the nascent creature, having escaped those integuments bind- 

 ing its parts compactly together, until liberated by birth. 



(2.) Planula, Corpusctditm, Spinula. — The originating being having 

 absorbed the pabulum provided for it by Nature, and having attained the 

 requisite degree of maturity in quiescence within the ovum, escapes from 

 it into active life, but still to undergo certain marvellous metamorphoses 

 in progress to equal perfection with its parent. 



Metamorphosis or transformation, of which we speak as familiarly as 

 if it were perfectly understood, is nevertheless a most obscure operation in 

 animal physiology. 



Can we say more than that it consists of the decay or diminution of 

 certain parts of a living organism, and the unequal or disproportionate 

 evolution of others in comparison to the rest ? Something, therefore, — a 

 certain form will ensue, which has not hitherto been. 



The earliest sensible alteration in the ovum of the hydraoid and of 

 some other zoophytes, is its elongation into an entire animal, rudely re- 

 sembling the general outline of a tadpole, which we have designated a 



