CYTOPLASM 



195 



OH 

 Nucleus 



Optically 



homogeneous 

 zone 



Chondrisomes 



Yolk 



nomenon in a liquid. This, however, is contradicted by the following 



interesting and extremely remarkable fact: by further centrifuging, 



the egg cell can be separated into two halves, as indicated in Fig. 1 1 3 



by a line. In this process a clear part containing the nucleus and a 



pigmented part without nucleus are formed. Both can be inseminated 



and are then capable of division (E. B. Harvey, 



1933), and the part which does not contain the 



nucleus may sometimes be induced to divide 



without any nucleus. E. B. Harvey (1936) 



concludes from this: "It must therefore be the 



'ground substance' which is the material for 



development - the matrix which is not moved 



by centrifugal force and which, in the living 



egg, is optically empty". Lehmann (1945) 



points out that in the outer layers of the 



Tuhifex and the sea-urchin egg, there must 



be a morphogenetic pattern, which cannot 



be destroyed by centrifugal forces. 



In other words, the method of centrifuging 

 also leads to the conclusion that an invisible 

 ground framework must exist, which is torn 



apart in the centrifuge by the oil droplets, yolk and pigment particles 

 respectively, as a result of their different weights. The microscopically 

 visible particles must move in the opposite direction through the 

 meshes of this framework without damaging it seriously, seeing that 

 division and growth of the plasmic fragments separated by centrifu- 

 gation still takes place afterwards. For this reason the framework must 

 either possess very coarse meshes, or else it must be possible for the 

 important molecular groupings, whose mutual positions have been 

 altered by centrifugation, to be restored to their original arrangements. 



By centrifuging, the invisible cytoplasmic frame is orientated, for 

 the drawn-out plasmatic neck shows positive birefringence with respect 

 to the axis (Pfeiffer, 1941b). Its reticular structure must possess an 

 unexpected mechanical stability, for A.scaris eggs can stand centrifugal 

 fields of 950,000 times gravity for 10 hours or 400,000 times gravity 

 for 10 days (Beams, 1943), without dying or losing their normal 

 capacity for development, although, with the exception of the nucleus, 

 all components of the cell appear to be completely separated from the 



Pigment 



Fig. 113. Egg cell of Arbacia 

 punctulata after centrifuging 

 (from E.B.Harvey, 1936). 



