340 WHEELER. [Vol. IV. 



Like all the cells which they produce, the neuroblasts are 

 finally enclosed by the outer neurilemma. By the time the 

 embryo has undergone revolution and has enveloped nearly the 

 whole of the yolk, the ganglia are so crowded with cells that 

 the neuroblasts have very little space in which to proliferate. 

 They continue, however, to bud off new cells, notwithstanding 

 the short rows thus formed are forced to lie parallel instead of 

 at right angles to the surface of the ganglion. During this 

 process the neuroblasts gradually grow smaller, till they finally 

 become indistinguishable from their progeny, the ganglion cells. 



I would emphasize the definite number and arrangement of 

 the neuroblasts in XipJiidhmi, because I believe the eight rows 

 of the lateral cords to be the homologues of the two rows of 

 cells derived from the neuro-teloblasts in Annelids. The fact 

 that there are only two rows in Annelids, whereas there are 

 eight in XipJiidiimi, can constitute no very serious obstacle to 

 this homology, since we have only to suppose that in the anne- 

 lid-like forms from which the Tracheates are descended, the 

 pair of primitive neuro-teloblasts divided twice to form four 

 pairs of proliferating centres for the longitudinal rows of neuro- 

 blasts. It is not improbable that forms with more than a single 

 pair of neuro-teloblasts may yet be found among existing Anne- 

 lids. The homology here advocated is rendered more probable 

 by the fact that the embryo XipJiidiitm does not conform to 

 Graber's "law" of metameric segmentation, but, like Myrio- 

 pods and Annelids, grows by the intercalation of segments in 

 front of the anal plate. The neuroblasts in the proliferating 

 zone at the end of the body are probably true teloblasts, since 

 they bud off the neuroblasts of the different segments. They 

 may be called primary neuro-teloblasts to distinguish them 

 from the secondary neuro-teloblasts that produce the strings 

 of ganglion cells. 



It is more difficult to account for the interrupted series of 

 median cord neuroblasts. Do they represent the remnants of 

 a single unpaired row or of a pair of rows } Are they seconda- 

 rily derived from one or both of the inmost rows of lateral 

 cord neuroblasts (5^), or have they had an independent origin } 

 These are questions which I cannot yet answer. The interrup- 

 tions in the median series are probably due to the formation 

 of the commissures ; primitive conditions prevailing only in 



