FIBRILLAR SYSTEMS IN THE MITOTIC APPARATUS 175 



the two species of sea urchin may be considered real. Quantitative 

 measurements on various fractions from living material, during the 

 time course from fertilization to cleavage, combined with parallel 

 qualitative immunochemical determinations, could be expected to 

 improve our understanding of the proposed interrelationships among 

 the states of the precursor-1 component. 



By way of summary, we can say that the unfertilized egg is 

 endowed with the molecular subunits that have been identified as 

 important structural elements in the mitotic apparatus. The forma- 

 tion of the mitotic apparatus may then be considered the result of 

 the spatial rearrangement of the precursor-1 and precursor-2 com- 

 ponents into the highly oriented aggregate, the mitotic apparatus. 

 It has been further speculated that the main reservoir of these com- 

 ponents in the unfertilized egg resides in existing subcellular moie- 

 ties—the microsomes, mitochondria, and the structural elements of 

 the yolk particles— and that the soluble fraction represents an inter- 

 mediate form in the mobilization of the insoluble fraction into the 

 mitotic apparatus. 



Before we can fully accept the precursor concept for the origin 

 of the mitotic apparatus in the sea urchin, it must be demonstrated 

 that tlie precursor molecules that appear in the mitotic figure are 

 indeed the same ones that occur in the unfertilized egg. 



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