Determination of Location of Central Zone 75 



In the pluricellular animals we can start out with the 

 simple supposition that if a central zone of development 

 is present, it is probably that in which the blastomeres 

 multiply with unlike rapidity ; that is, it will be constituted 

 by these blastomeres which multiply most rapidly. For 

 this greater rapidity would in the majority of cases 

 indicate a greater vitality or energy, which may be 

 produced by a richer protoplasm or by any other special 

 condition favoring the nutrition. And this greater 

 energy would bring it about that the cells possessing it 

 would win the upper hand over the others. 



"The zone," writes Oscar Hertwig, "where the 

 smallest embryonal cells lie, which are also those which 

 divide most rapidly, becomes the place of gastrular 

 invagination ; it becomes something like a fixed center of 

 crystallization for the animal development." 48 But these 

 blastomeres are those which, in the vertebrates for 

 example, later constitute the medullary tube and 

 afterward the spinal cord. 



As we shall see, everything seems to lead to the con- 

 clusion that in animals with specialized nervous systems 

 the central zone is constituted by the least differentiated 

 part of the nervous system itself; in the vertebrates 

 probably by the innermost periependymal part of the 

 spinal cord. This part after completion of its activity in 

 determining development would, on account of the so- 

 matic function pertaining to it, likewise constitute the 

 place where the infinitely manifold nervous activities of 

 all the rest of the nervous system or rather of the whole 

 organism are faintly re-echoed. 



As we have already said, this location of the central 



48 Oscar Hertwig: Zeit- und Streitfr?gen der Biologic. Heft. 2. 

 Mechanik und Biologic. Jena, Fischer, 1897. P. 180. 



