CONCLUSION 209 



The conversion, therefore, of a system of protoneu- 

 rones without synaptic interruptions into one of neu- 

 rones related through synapses is the essential step in 

 passing from a nerve-net to its derivatives, a true central 

 nervous organ. In animals in which nerve-nets prevail, 

 the lower invertebrates, the embryonic cells that give rise 

 to their protoneurones are in the course of their devel- 

 opment near neighbors so that the intimate relations 

 which they bear to one another in the final net may after 

 all be an expression of that closeness of relation that 

 has existed between them from their embryonic states 

 onward. It is not impossible that protoneurones that are 

 united with each other in the nerve-net retain their 

 strands of connection from embryonic stages when in the 

 course of cell division they were never really entirely 

 separated. 



With the neurones of the differentiated central nerv- 

 ous system the case is usually very different. In the em- 

 bryo of one of the higher animals the neurones are repre- 

 sented by cells without processes, the neuroblasts, which, 

 as they differentiate into neurones, do so in part by the 

 production of extensive processes. In this way two neu- 

 roblasts that lie at almost the opposite ends of the devel- 

 oping nervous system may by means of their processes 

 connect directly with each other as neurones in the adult. 

 Thus certain neuroblasts in the developing cerebral cor- 

 tex of mammals differentiate into neurones by throwing 

 out processes that eventually reach in a remote part of 

 the spinal cord motor neurones whose neuroblasts could 

 have had no possible connections with those in the cortex. 

 Both sets of cells are in the beginning widely separated 

 and their final union depends upon the cell processes that 

 have grown from one set to the other. How intimate this 



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