625 



two smaller white cells were cut ofi above. In other eggs the 

 cleavage was more irregular. In later stages the upturned white 

 portions of the eggs were composed of smaller cells than the lower 

 dark hemisphere. The two embryos which developed from these eggs 

 were of half size having both right and left medullary folds 

 and are unquestionably perfect embryos of half size. One of 

 these showed posteriorly a spina befida. The conclusion follows that 

 the position in which the egg is placed after the operation is 

 an important factor in deteimining whether a half-embryo or 

 a whole-embryo of half size will develope from one of the first 

 two blastomeres. 



The results of both Roux and Hertwig are true in so far as 

 they apply to the embryos described by each, and the conclusions of 

 each are wrong in so far as they deny the poäfeibility of the results 

 of other. Both authors have overlooked the essential condition of the 

 experiment ! 



Conclusions. 



A careful examination of the papers of Roux ^) and Hertwig *) 

 shows, that each author has occasionally seen both half-larvae and 

 whole larvae of half size. Nevertheless Hertwig has almost totally 

 ignored the one clear case of a half-larva which he obtained and 

 Roux has interpreted the whole larvae, which apparently he has seen, 

 as hemiembryones anteriores, 



Hertwig's description of his half-larva is as follows : "Der letzte 

 endlich aus der Reihe dieser Embryonen (Taf. XLIII, Fig. 1) ist von 

 besonderem Interesse, weil die Zerstörung der Dottersubstanz auf der 

 einen Seite so weit ausgedehnt ist, daß sich an ihr ein Urmundrand 

 überhaupt nicht hat bilden können. Links sehen wir eine halbe Medullar- 

 platte, Chorda (ch) und mittleres Keimblatt entwickelt" ^). Roux has 

 with justice said in respect to this larva: "Demnach hat 0. Hertwig 

 also einen richtigen Hemiembryo lateralis mit bloß dem linken Me- 

 duUarwulst, bloß linkem Mittelblatt, linker Darmhöhle erhalten, .... 

 dieser Hemiembryo würde mit seinem einen Medullarwulst eine sehr 

 in die Augen fallende Widerlegung von Hertwig's Folgerungen ab- 

 gegeben haben." 



On the other hand Roux has to all appearances found whole 

 embryos of half size which he has interpreted as follows : "Da unsere 



1) Anatomischer Anzeiger, Bd. IX, Nr. 8. 



2) Archiv f. mikroskop. Anatomie, Bd. XLII. 



3) Pag. 762. 



