OP THE NISUS FORMATIVUS. 377 



into the cavity of the uterus, (527, 533, 551.) require a 

 certain period for becoming intimately mixed, acted 

 upon, and matured; that, after this preparatory stage, 

 the nisus formativus is excited in them, vivifying and 

 shaping the hitherto shapeless spermatic matter partly 

 into the beautiful containing ovum (565) and partly into 

 the contained embryo ; (569) and that this is the reason 

 of our inability, notwithstanding the present perfec- 

 tion of optical instruments, to discover, during the first 

 weeks after conception, any thing more than shapeless 

 fluids, without the faintest trace of the form of an 

 embryo, which, however, about the third month, sud- 

 denly, as it were, becomes observable. 



589. We should exceed the limits of these institu- 

 tions, were we to adduce many of the arguments which 

 may be drawn from facts, to illustrate, as, in our 

 opinion, they most clearly do, the great influence of 

 the nisus formativus in generation. We will, however, 

 venture to mention, as briefly as possible, a few, 

 whose weight will, on a little close reflection, be suf- 

 ficiently evident. 



590. Such, in the history of hybrid animals, is the 



2. The word nisus I have adopted chiefly to express an energy truly vital, 

 and therefore to distinguish it as clearly as possible from powers merely 

 mechanical, by which some physiologists formerly endeavoured to explain 

 generation. 



3. The point on which the whole of this doctrine respecting the nisus for- 

 mativus turns, and which is alone sufficient to distinguish it from the vis plastica 

 of the ancients or the vis cssentialis of the celebrated Wolff and similar hypo- 

 theses, is the union and intimate co-e.rertion of two distinct principles in the 

 evolution of the nature of organised bodies, — of the PHYSICO-MECHANICAX 

 with thPpurely teleological, — principles which have hitherto been adopted 

 but separately by physiologists in framing theories of generation. 



