EXPERIMENTS ON OVA 11 



complete embryos, and even larvae in some cases. Although 

 these embryos contained all the ordinary tissues and parts 

 found in embryos produced from the ordinary fertilised 

 ovum, they were smaller in size. Thus embryos produced 

 by shaking the cells apart in the two-cell stage produced 

 embryos half the normal size. Those shaken apart when in 

 the four-cell stage gave rise to embryos a quarter of the 

 normal size ; and so on up to the sixteen-cell stage, which is 

 the latest period at which this experiment has hitherto been 

 successful with any animal, where the embryos, although 

 perfect, were only one-sixteenth of the normal size. These 

 experiments seem to prove that the characters cannot be 

 represented by entities that are distributed in a selective 

 manner among different cells during the process of develop- 

 ment, as is assumed by the Roux-Weismann theory. If it 

 were so, this selection of different entities must begin at the 

 first cell division ; but it has been proved by experiment, 

 that even when the sixteen-cell stage has been reached, each 

 of the sixteen cells possesses within itself the power of pro- 

 ducing, not only the tissues which it would produce under 

 normal conditions were the ovum left to itself to develop, 

 but, when separated from its fellows, also all those tissues 

 that would have been produced under normal conditions by 

 the other fifteen cells. 



Against the idioplasm theory in its original form there 

 is no evidence whatever. With such facts as are known 

 to us, it is quite reasonable to assume the existence of a 

 substance somewhere in the fertilised ovum which has the 

 property or characteristic of developing along particular 

 lines, and of producing particular kinds of cells. Such a 

 substance must differ in different kinds of organisms, and 



x. 19, 1895; Wilson, E. B., "On Cleavage and Mosaic Work," Arch. f. 

 Entwick., iii. 1, 1896; Zoja, R., " Sullo sviluppodei blastomeri isolati dal- 

 le nova di alcune Meduse," Arch. /. Entwick., i. 4, ii. 1 and 4, 1895; Her- 

 litzka, A., " Contribuzione allo studio della capacita evolutiva dei due primi 

 blastomeri nell' uove di Tritone," Arch. f. Entwick., ii. 3, 1895; Crampton, 

 H. E., "The Ovarian History of the Egg of Molgula," Journal of Morphology, 

 XL. Supplement, 1899. 



