7 g THE E VOL UTION OF SEX. 



adult organisms. Conveniently in the ordinary frog-spawn from the 

 ditch, we can read what was so long a riddle — how development pro- 

 ceeds by successive cell-divisions and by arrangement of the multiple 

 results. Readily seen in many instances, it is true of all cases of 

 ordinary sexual reproduction, that the organism starts from the union 

 of two sex-cells, or that it is with the division of a fertilized ovum that 

 development begins. 



This profound fact, technically known as the "ovum-theory," has 

 been not unjustly called by Agassiz ' ' the greatest discovery in the 

 natural sciences of modern times." We shall the better realize the 

 magnitude of the difference which its recognition has introduced into 

 biology if we briefly review the history. 



II. The History of Embryology, Evolution, and Epigen- 

 esis. — The development of the chick, so much studied in embryo- 

 logical laboratories today, was the subject of inquiry two thousand 

 years ago in Greece. Some of the conspicuous marvels of reproduc- 

 tion and development were persistently but fruitlessly speculated about 

 ' hroughqut centuries. It was only during the scientific renaissance of 

 "enteenth century that the inquiry became more keen and 

 ane, and began to rely to some extent at least on genuine 



^rvation. 



(a) Harvey (1651), with the aid of magnifying glasses (pcr- 

 specillce), demonstrated in the fowl's egg the connection between the 

 cicatricula of the yelk and the rudiments of the chick, and also 

 observed some of the stages of uterine life in mammals. More import- 

 ant, however, were his far-sighted general conclusions : ( 1 ) that every 

 animal was produced from an ovum {ovum esse primordium commune 

 omnibus animalibus); and (2) that the organs arose by new formation 

 (epigenesis), not from the mere expansion of some invisible preforma- 

 tion. In this generalization, without however any abandonment or 

 the hypothesis of spontaneous generation of germs, he strove, as he 

 said, to follow his master Aristotle, and was in so doing as far ahead 

 of his contemporaries as a strong genius usually is. Before Harvey, 

 the observational method had indeed begun. Thus, as Allen Thom- 

 son notes, Volcher Coiter of Groningen (1573), along with Aldrovan- 

 dus of Bologna, had watched the incubated egg in its marvelous 

 progress from day to day. Fabricius ab Aquapendente (1621) had 

 also studied the changes in the incubated egg, and the stages of the 

 mammalian foetus. In keenness of vision, Harvey was far ahead 

 of any of these. 



(b) Malpighi (1672), using a microscope with phenomenal skill, 

 traced the embryo back into the recesses of the cicatricula or rudiment, 

 but again missed a magnificent discovery, and supposed the rudiments 



