The mesenterial Filaments of the Alcyonaria. 1 1 



The cells of the filament retaiu their characters in later stages, are 

 uuiform from one eud to the other, and always may be followed directly 

 up into the inner wall of the Oesophagus. Hence the re can be no 

 doubt that the dorsal filaments of the egg-embryo are 

 morphologically nothing but narro w prolongations, along 

 the septa, of the inner layer of the Oesophagus; and the 

 latter is agreed by all observers to consist of ectoderm 

 i n V a g i n a t e d from the e x t e r i o r . 



Turning uow to the development of the buds, we find precisely 

 similar facts. In a very young bud of FunicuUna^ for instance (fig. 13, A) 

 the dorsal filaments, d.f., appear as slight knob-like outgrowths from 

 the inner layer of the Oesophagus. Everything I have said of the young 

 dorsal filaments of Clavularia will apply equally well to the present 

 case, and need not be repeated. A later stage in the bud of Alcyonium 

 is shown in fig. 10. The dorsal filaments, r//., are considerably longer, 

 and the six other filaments, one of which is shown at e/., have ap- 

 peared. The histologieal structure is the same as in Clavularia in every 

 detail, except that the contrast bctween the ectodermic cells of the fila- 

 ment and the entodennic cells of the septum is even more striking. 



A still later stage in the bud of Funiculina is shown at B., fig. 13. 

 The ectodermic filaments, d.f. , now extend to the bottom of the stomach- 

 cavity, but their structure isquite unaltered and the characteristic colum- 

 nar cells may be traced up without a break into the inner layer of the 

 Oesophagus , so that it is not possible to say where the filament begins. 

 An older bud is shown at C, fig. 13, and a still later stage of Kojiko- 

 helcmnon in fig. 5. In the latter, especially, the dorsal filaments have 

 attained an enormous lengtli , but the histologieal characters are un- 

 changed. In sections stained with Grenacher's alum-carmine or with 

 haematoxylin , the contrast in appearance between these filaments and 

 the entodermic structures is most striking. The former appear of an 

 intense blue or purple color , from their closely-crowded deeply-stained 

 nuclei , whereas the entodermic structures are pale , granular , usually 

 much pigmented, and contain a much smaller number of nuclei. These 

 facts show beyond the possibility of doubt that inthebuds, asiuthe 

 primary polyp, the dorsal filaments are ectodermic struc- 

 tures and are morphologically downgrowths from the 

 stomodaeum. 



If we examine the rudimentary individuals or zooids of the Penna- 

 tulida we find quite similar facts. It is a well known discovery of 

 KöLLiKER that the zooids of mauy species possess a single pair of fila- 



