906 



SCIENCE 



[N. S. Vol. XXVII. No. 702 



stances") within the egg, and which de- 

 termines the axis of embryonic develop- 

 ment. This organization of the ground 

 substance is not substantially altered by 

 centrifuging, though it may be temporarily 

 distorted thereby. 



These conclusions depend upon the fol- 

 lowing considerations and observations: 



I. Concentric Organization. — If the 

 ground substance be perfectly homo- 

 geneous, granules moving under the influ- 

 ence of centrifugal force will pass in 

 straight lines centrally and distally, ac- 

 cording to their respective specific gravi- 

 ties, and will not be held in any inter- 

 mediate position. Aggregation of gran- 

 ules in definite configurations or inter- 

 mediate positions after centrifuging are 

 prima facie evidence of differences of 

 resistance of the ground substance. 



Such definite configurations are found 

 in the egg of Chcetopterus after the action 

 of low centrifugal powers with perfect 

 constancy ; and these together with the evi- 

 dence derived from study of the normal 

 structure enable us to infer the following 

 concentric organization of the ground sub- 

 stance: (1) An ectoplasmic layer of con- 

 siderable density, open at the animal pole ; 

 (2) an intermediate more fluid layer which 

 is especially developed at the animal pole, 

 where it comes to the surface; (3) a ring 

 of greater density, enclosing (4) a large 

 subcentral, more fluid mass. The last cor- 

 responds in position and approximately in 

 size with the original location of the large 

 germinal vesicle. 



As a consequence of this organization of 

 the ground substance, the small basophile 

 granules within the central, more fluid 

 area aggregate in the dense ring bounding 

 it, when low centrifugal powers are used. 

 This aggregation is situated towards the 

 axis of the centrifuge, as the basophile 

 granules are of low specific gravity, and is 

 lunate in cross-section ; it contrasts vividly 



with the surrounding parts of the egg after 

 staining in strong basic dyes, such as 

 thionin. Higher powers of the centrifuge 

 drive such granules through the dense ring 

 into the "clear zone" which contains the 

 basophile granules from other parts of the 

 egg. The acidophile granules also show 

 typical configurations. The central, more 

 fluid mass is practically emptied of gran- 

 ules when low centrifugal powers are used, 

 because those within it are driven into 

 the dense ring bounding it and those with- 

 out do not penetrate the dense ring to 

 enter it. 



II. Polar Organization.— The polarity 

 of the ovum before and after centrifuging 

 appears to be the same whether the karyo- 

 kinetic spindle retains its original position 

 or not. This is inferred from the fact 

 that the polar axis of centrifuged eggs (as 

 determined by the position of the polar 

 bodies appearing subsequent to centrifu- 

 ging, and the later development) bears no 

 constant relation to the axis of stratifica- 

 tion of granules produced by centrifuging. 

 Therefore the position of the granules of 

 the egg does not determine its polarity. 

 The latter may, therefore, be either a per- 

 sistent feature of the ground substance, or 

 may be determined by whatever point on 

 the surface the maturation spindle hap- 

 pens to reach if displaced by centrifuging. 

 The latter proposition appears highly im- 

 probable because (1) the polarity of 

 ovocytes with intact germinal vesicle does 

 not appear to be determined by the point 

 on the surface to which the germinal 

 vesicle may be thrown by centrifuging 

 (limited number of observations of au- 

 thor) ; (2) the position of the maturation 

 spindle displaced by centrifuging is con- 

 stant, viz. : in the clear band, but the polar 

 bodies are frequently formed outside this 

 band; (3) in other animals no cases are 

 known where the position of the nucleus 

 has been shown to determine polarity, but, 



