SECT. 5] IN ONTOGENESIS 837 



hand there was little difference between the viscosity of the white in 

 incubated fertile and infertile eggs. The whole question merits more 

 study, in view of the obvious importance possessed by the mechanism 

 of preparation of the first pabulum of the embryo. For the frog's 

 egg, rhythms of viscosity have been reported by Odquist, who, 

 however, used a centrifugation method followed by observation on 

 pigment distribution. 



As regards the protoplasmic viscosity of alecithic eggs, Heilbrunn 

 has worked on those of the clam, Cumingia, and the sea-urchin, 

 Arbacia punctulata, using the centrifuge method. In both cases there 

 was a maximum viscosity at 15°, from which the values fell away 

 on both sides. Pigorini subsequently obtained very similar results for 

 the expressed juice of silkworm's eggs. 



Heilbrunn in 1920 also studied the effect of anaesthetics upon 

 viscosity in eggs, and in 1 9 1 5 the effect of hypertonic solutions ; the 

 original papers must be consulted for the details, 



Seifriz has also done much along these lines, using for the most 

 part the method of microdissection. In many cases this method has 

 given diametrically opposite results to those obtained by other 

 techniques, especially the centrifuge, and has occasioned some 

 polemics which need not be described in detail. Seifriz maintained 

 that the protoplasm of Echinarachnius eggs was immiscible with water, 

 and allotted a definite degree of viscosity to the cytoplasm at different 

 stages during cleavage, etc. A large element of the subjective must 

 have entered into the determination of these values, and where they 

 have conflicted with those obtained by the centrifuge method they 

 have been abandoned by most biologists, who prefer the more ob- 

 jective technique. Seifriz's viscosity values were as follows: 



On this scale the vv of mature unfertiUsed eggs of Echinarachnius 

 and Tripneustes was 7. This agreed with the microdissection estimates 



