No. 3.] STUDIES ON THE HETERONEMERTINI. 401 



These figures show: (i) that the number is not equal in both 

 chords, and (2) that there are about three or four times as 

 many cells on the dorsal sides of the chords as there are on 

 their ventral sides. 



The neurochord cells are absent in the oesophageal region, 

 and also in the caudicle; thus both ends of the lateral chords 

 are devoid of them.^ They increase in number from the ante- 

 rior to the posterior end of the chord, and, in which I can also 

 corroborate Burger's observations, only dorsally situated cells 

 occur in the region just anterior to the caudicle. They are 

 always situated medially, close to the circular musculature, as 

 Burger states, but are frequently more or less lateral in posi- 

 tion. The cells in the left chord are seldom paired with those 

 in the right, though it is not infrequent, through a short extent, 

 for those of the one to alternate with those of the other. But 

 it is more usual for three or four to succeed one another in 

 the one chord, while the corresponding portion of the opposite 

 chord is devoid of cells, this order being reversed further on. 

 Then, too, in the same chord dorsal and ventral cells do not 

 alternate, and seldom occur together on one section (I found 

 only two sections of a ventral chord, each of which contained a 

 dorsal and a ventral cell) ; similarly, two cells in the same sec- 

 tion, both on the dorsal side of a chord, I found only twice, 

 and have figured one of these cases (Fig. 32). With these 

 exceptions, the neurochord cells were separated, as a rule, by 

 considerable; though irregular, distances, so that in a series of 

 forty consecutive sections from the middle of the body, seldom 

 were more than six neurochord cells to be observed in both 

 nerve chords. 



The only apparent regularity in their distribution is that, in 

 certain portions of the chord at least, zones may be distin- 

 guished in which they follow one another in comparatively close 

 sequence, alternating with areas (of approximately equal extent) 

 where they are much less abundant. These alternating areas 



1 I have shown previously ('97) that in the caudicle the fibrous core of the 

 lateral chord is directly enveloped, on all sides except the median, by a mass of 

 true mesenchym cells, and that no ganglion cells accompany the chord in this 

 region. 



