360 A. A. W. nUBREOHT 



rod-like organ, the other a hollow tube. However, at the very 

 earliest stages of its formation the notochord of the primitive ver- 

 tebrates (cf. Hatchek's ^ Development of Amphioxus') possesses a 

 central groove, which is a derivate of the archenteron, and which 

 only secondarily, in accordance with the further differentiation 

 of the tissues of the notochord, obliterates.^ 



The ulterior difference in histological structure of the one, a 

 cellular tissue eminently vacuolar, and of the other: the cellular 

 lining and fluid contents of a tube, the cavity of which does, as 

 a rule, not obliterate, is, however, no serious objection to their 

 eventual homology. In more than one instance moilern mor- 

 phology recognises solid cellular strings to be homologous with 

 others containing a cavity inside them. 



The different degree in which muscular tissue takes part in 

 the constitution of the proboscidian sheath (figs. 16 — 18) must 

 of course not be overlooked, the more so as it is entirely absent 

 in the notochord and its envelopes. However, in Nemertines 

 this muscular tissue can be shown to be most closely related to 

 the function of the proboscis, and in fact to be sometimes ex- 

 ceedingly reduced. For this reason its importance as a point 

 of comparison must not be over-rated. 



All these differences are in the last instance due to the different 

 significance in the animal economy which has in the two groups 

 been attained by this organ. In the Vertebrates this central 

 rod-like structure, sustaining the mesoblastic somites in their 

 progressive development, has a significance as a temporary axis, 

 round which these processes take place. Its important cha- 

 racter as a primitive, i.e. as an ancestral organ, is recognised 

 notwithstanding, or rather just because of, its gradual disap- 

 pearance in the adult forms of the higher groups, where its 



' I must not omit to call attention to certain papers in the volume for 18S2 

 of the 'Archiv fiir Anatomic und Physiologic.' They came into my hands 

 after the completion of my MS. The one is by Lieberkiihn, " Ueber die 

 Chorda der Siiugclhieren ; " the other by Braun, " Entwickelungsvorgiiuge am 

 Schwanzende der Siiugethieren." Both naturalists have succeeded in demon- 

 strating that in diflerent regions of the body the notochord is at first a 

 hollow, tubiform structure (fig. 11). Braun found the same in 

 birds. XoUikcr, Sliahl, and others have lately come to similar results ! 



