50 REPRODUCTION AND LIFE HISTORY OF ANIMALS. 



generation, and the next, and the next, and all future generations. 

 Germ lay within germ, preformed in transparency, and in successively 

 smaller miniature, after the fashion of an infinite juggler's box. We 

 laugh at this, but we need not laugh too much, for the preformationists, 

 though wrong and crude in their facts, were right in two of their ideas, 

 — that the germ contains the potentiality of a future organism, and that 

 it has relations, not only to the animal into which it develops, but also 

 to generations following. (See p. 71.) 



In the middle of the seventeenth century, however, Harvey had 

 reached conclusions which might have saved much blundering. Study- 

 ing the development of the chick, — as Greek naturalists had tried to do 

 wellnigh two thousand years before, as we are doing still in our embrj-o- 

 logical laboratories, — Harvey inaintained that every animal was produced 

 from an ovum [oviun esse p7'in!ordiiivi eojnnnme omnibus aninialibus)^ and 

 that organs arose by new formation (epigenesis), not by the expansion or 

 " evolution " of some invisible preformation. 



But the great champion of epigenesis was Caspar Friedrich Wolff, 

 who, in his doctorial dissertation of 1759, traced the chick back to a 

 layer of organised particles (the familiar cells of to-day), in which there 

 was no likeness of the future embryo, far less of the adult. 



Wolff was long in finding successors, but in 1824 Prevost and Dumas 

 described the division of the ovum ; in 1827 Von Baer discovered the 

 mammalian ovum ; while Wagner, Von Siebold, and others elucidated 

 the real nature of the spermatozoon. 



A great step was made in 1838-9, when Schwann and Schleiden 

 formulated the "cell theory," according to which every organism is 

 made up of cells, and starts from a cell. From this date modern em- 

 bryology began. 



Sexual Reproduction. 



There is apt to be a lack of clearness in regard to se.xual 

 reproduction, because the process which we describe by 

 that phrase is a complex result of evolution. It involves 

 two distinct facts : — {a) the liberation of special germ cells 

 from which new individuals arise ; (/;) the occurrence of two 

 different kinds of germ cells — ova and spermatozoa, which 

 come to nothing unless they unite (fertilisation). Further- 

 more, these dimorphic reproductive cells are produced by 

 two different kinds of individuals (females and males), or 

 from different organs of one individual or at different times 

 within the same organ (hermaphroditisin). 



It is conceivable that organisms might have gone on 

 multiplying asexually, by detaching overgrown ponions of 

 themselves which had sufficient vitality to develop into 

 complete forms. But a more economical method is the 

 liberation of special germ cells, in which the qualities of the 



