382 BERTRAM G. SMITH 



1. Relation to the excentricity in the superficial cleavage pattern. 

 It is a very natural supposition that the excentricity in the super- 

 ficial cleavage pattern is merely the outward expression of the 

 excentric development of the micromeres revealed by the study of 

 the internal structure. To test this hjq^othesis, sixteen preserved 

 eggs in stages 5 to 8, inclusive, were spht (with a razor blade) 

 along the axis of excentricity in the superficial cleavage pattern 

 and the internal structure examined with a lens. In every egg 

 but one the roof and lateral walls of the blastocoele showed ex- 

 centric development of the micromeres after the fashion pre- 

 viously observed in serial sections, and in twelve eggs the median 

 plane of excentricity in the internal structure coincided with the 

 plane of splitting. 



The point was further investigated bj^ means of specially 

 prepared serial sections. Nineteen eggs in stages 6 to 8, in- 

 clusive, showing marked excentricity in the superficial cleavage 

 pattern, were oriented in parafF.n, and sectioned in planes parallel 

 to the axis of excentricity. In seventeen eggs the meridional 

 sections showed approximate coincidence in the direction of 

 excentric development of the micromeres as manifested by exter- 

 nal and internal features, respectively, while in the two re- 

 maining eggs the meridional sections gave no evidence of excentric 

 development. 



Therefore we conclude that the excentricity manifested by the 

 internal structure of the early blastula is definitely correlated with 

 the excentricity in the superficial cleavage pattern; these two 

 features are different aspects of the same thing. The experi- 

 mental evidence has shown that there is no constant relation be- 

 tween the axis of excentricity in the superficial cleavage pattern 

 of the early blastula (stages 5 to 7, inclusive) and the median 

 plane of the gastrula; we must now accept the same conclusion 

 for the excentricity in the internal structure. 



Incidently, the study of sections shows why, in stages 8 and 

 9, the excentric development of the micromeres is not so clearly 

 expressed by the superficial cleavage pattern as it is by the deeper 

 structure. In these stages there occurs a rather uniform flatten- 

 ing of the superficial layer of micromeres, which masks the 



