Manchester Memoirs, Vol. I. (1906), No. % ^i 



of the fertilized ovum — is itself a heritage from a long 

 line of ancestors, each individual will of necessity repeat 

 in its ontogeny the history of its descent. If, on the 

 other hand, there is no necessary constancy, even within 

 the limits of a single species, in the fate of cells of identical 

 origin, such a regularity need certainly not be claimed for 

 an entire class. 



The question is thus narrowed down to a perfectly 

 plain issue, which may be brought to the test of experi- 

 ment ; with the preformationist hypothesis germ layer 

 theories must stand or fall. 



We owe the pressure experiments, to which allusion 

 has been made, to Driesch^^ and Hertwig.^® The former 

 showed in Echinus, the latter in the Frog, that by this 

 means the nuclear spindles could be displaced, and the dis- 

 arranged nuclei made to pass into other than their normal 

 blastomeres, without affecting the normality of subsequent 

 development. In Hertwig's experiments, it is interesting 

 to observe, nuclei pass into vegetative which are ordinarily 

 allotted to animal cells, and vice versa. Wilson's^^ experi- 

 ments on Nereis are of a like nature. By pressure the 

 egg is made to segment into a flat plate of eight cells, 

 all of which, when the pressure is released, behave as 

 macromeres. Eventually cells are found in the lining of 

 the gut, which ought to have contributed to the ectoderm of 

 the first quartette. 



The results of the experiments on the development of 

 isolated blastomeres fall into a regular series. At one end 

 of this stand the Vertebrates and Amphioxus, the Asci- 

 dians^^" and the Coelenterates ; here an isolated blastomere 



^^ Zeitschr. -iviss. ZooL, vol. 55, p. I — 62, 1893. 



^* Arch. mikr. Attat., vol. 42, p. 662 — 794, 1893. 



59 Arch. Ent.-mech., vol. 3, p. 19—26, 1896. 



^^"■'Dnt?,ch,ll., Arch. Ent.'iJieck., vol. i, p. 398— 411, 1895. E.G. 

 Conklin has, however, recently shown that in Cynthia there is a pre- 

 localization of definite organ-forming substances in the egg {Jotirn. Exp. 

 ZooL, vol. 2, p. 145 — 221, 1905). 



