214 Robert Thompson Young, 



great that, while I consider this origin of ganglion cells a highlj^ 

 probable one, I am nevertheless loth to claim it positively, 



Eegarding the migration hypothesis, all the evidence at my 

 command points against it. In no instance in the histo- 

 genesis of Cysticercus pisiformis is there migration 

 of cells or tissues relative to other cells or tissues. 

 The reader may object that the backward growth of the base of 

 the scolex rudiment, forming the tongue of muscular tissue which 

 projects into the bladder cavity, constitutes such an instance. But 

 this I deny, inasmuch as this tongue is formed, not by the migration 

 of pre-existent tissue, but by an accretion to the scolex of newly 

 developed tissue. Arguing against the probability of this hypoth- 

 esis is the fact that there are nowhere evident single cells or 

 groups of cells apparently unconnected with the surrounding tissues 

 and hence possibly representing migrating bodies. Were, for in- 

 stance, the lateral cephalic ganglia generative centers for ganglion 

 cells, which later migrated thence to their definitive centers along 

 the cords, we should expect to find a continuous string of such 

 cells in progressive stages of development passing backward in or 

 along the nerve cords to the developing proglottids. And one does 

 find numerous ganglion cells lying along the cords in the neck region 

 of both the larva and the adult. But these cells are so interspersed 

 with parenchyma elements as to render it very improbable that 

 they have migrated into the positions which they occupy. Only oc- 

 casionally do we find these cells within the cord itself and then 

 their fibres form a part of the fibrous tissue of the cord in such a 

 way as to i-ender in this instance also the theory of a migratorj^ 

 origin very improbable. 



The existence of several centers of development is proven by 

 the occurence of embryonic ganglion cells at many widely separated 

 points along the cord as well as by the discovery in certain larvae that 

 different parts of the nerve trunks develop separately from one 

 another. But it is highly probable that the nerve cells developed 

 at these centers maintain their positions relative to surrounding 

 tissues, their gradual separation from one another being due to the 

 growth of the intervening tissues and not to any active migration 

 on the part of the cells themselves. Moreover these various centers 

 are not sharply delimited from one another. 



It has already been shown in the description of the development 

 of the lateral cephalic ganglia, that the second hypothesis is correct 



