Embryology. 129 



be established in regard to fertilization, Virchow's con- 

 clusion led on to two others of fundamental importance. 

 The first of these was the conception of genetic con- 

 tinuity that the ovum was derived by continuous 

 cell-lineage from the fertilized ovum of the previous 

 generation, and bears with it from the first an inherited 

 organization. We shall return to this conception when 

 we discuss Heredity; it is enough to notice here that 

 it is the starting-point for every modern theory of 

 development or inheritance, and removes the stumbling- 

 block which was fatal to all the early theories. The 

 apparently ready-made organization of the fertilized 

 egg-cell, involving all the potentiality of the future 

 organism, becomes less unintelligible when we recog- 

 nize that it is, in a sense, itself an antiquity, a link in 

 the continuous chain of germ-cells. We owe the first 

 clear presentment of this idea to Virchow's classic work 

 (1858). 



The second corollary is one of great interest, practi- 

 cally as well as theoretically. Since the researches of 

 O. Hertwig and others in 1875, it had been clear that 

 each parent contributes a single germ-cell to the forma- 

 tion of the offspring; but the masterly researches of 

 E. van Beneden (1883) showed that every nucleus of the 

 offspring may contain nuclear substance derived from 

 each of the parents, a conclusion which is visibly demon- 

 strable for a few of the first steps in cleavage. In fact, 

 Van Beneden to some extent proved what Huxley had 

 foreseen when he said in 1878: "It is conceivable, and 

 indeed probable, that every part of the adult contains 

 molecules derived both from the male and from the 

 female parent; and that, regarded as a mass of mole- 

 cules, the entire organism may be compared to a web 

 of which the warp is derived from the female and the 

 woof from the male ". 



To Van Beneden and Boveri we also owe the discovery 

 of the centrosomes small bodies which seem to play an 

 important part in the division of animal cells. They 

 have been much discussed of recent years, and there is 

 still great uncertainty in regard to them and their asso- 

 ciated attractive spheres. One of the best-substantiated 



(M523) I 



