140 BIOLOGICAL LECTURES. 



was a favorable one for testing the conclusions of Roux. An 

 ^gg provided with an adhesive disk, by means of which it is 

 held constantly in a fixed position, seemed to offer just the con- 

 dition required for testing the mosaic theory of cleavage. 



The results have been reported in Xht Jon 7'ua I of Morphology, 

 vol. V, p. 498. In only t/n^ee cases out of twenty-three was 

 there coincidence of the first line of cleavage with the median 

 plane of the embryo. 



In 1892 Born^ called attention to my paper and pointed out 

 what he regarded as the "sources of error" in the case. His 

 criticism was accepted by Roux,^ who adds : " Only slight errors 

 of experiment are required in order to obtain almost equally 

 incorrect figures in the frog's o-gg.'' 



Born doubts the validity of the results, because of the long 

 period of time that elapses between the appearance of the first 

 furrow and the outline of the embryo. During this period of 

 six days, slight changes may occur to account for the " abwei- 

 chende Resultat." Born also finds difficulty in accepting the 

 assertion that " the adhesion of the yolk to the egg-membrane 

 prevented rotation," and says that "such adhesion would cer- 

 tainly hold good only for the first stages." 



Weysse,^ in 1894, believing that if there was adhesion of the 

 yolk to the membrane it could be demonstrated in sections of 

 the Qgg, made a careful examination, but found no trace of any 

 such attachment. He concludes, with Born, that " during the 

 six days mentioned abundant opportunity is furnished for a 

 rotation of the yolk within the egg-membrane." 



These experiments have been twice repeated since '91, with 

 essentially the same result. In order that the possible sources 

 of error may be fairly estimated, it seems desirable, in giving 

 the results of my later experiments, to repeat and perhaps 

 expand the brief description of the toadfish Qgg given in '91. 



The egg is large, — 5 mm. in diameter, — and as the blasto- 

 disk always develops at the free pole of the egg, the early 

 cleavage lines can be observed with a hand-lens without dis- 



1 Merkel and Bonnet, Erg., Bd. I, p. 502. 



2 "Beitrag zur Entwickelungsnniechanik des Embryo," Aiiat., Heft 7, p. 313. 

 ^ Proc. of Amer. Acad., vol. xxx, p. 308. 



