Ot?IGlN ANb STt?UCTURE OF tHE HfeAb 



133 



nevropore 



pole and indeed, the Ascidians would not be the only sessile 

 animals which attach themselves to their substratum with 

 jhe animal pole. The same can be said of the sessile Coe- 

 lenterata. Cirripedia and Echinoderm-larvae. perhaps of more 

 forms also. At the fixation of the Ascidian larva the praeoral 



lobe swells up enormously and 

 then presents the aspect of a large 

 cavity bounded by ectoderm and 

 containing loose, scattered, meso- 

 derm cells. The fixing papillae 

 flatten out and give place to a 

 thickened disc of ectoderm at the 

 anteiior end of the praeoral lobe. 

 Mouth of Ascidians. — If our 

 conception of the praeoral lobe 

 and the neuropore in Ascidia is 

 correct, the mouth of the Ascidian 

 larvae can be homologous neither 

 to that of Amphioxus nor to that 

 of Craniates, for it opens at a 

 place corresponding exactly to the 

 I europore.theold mou'h. Accord- 

 ing to Huntsman (I913)a con- 

 siderable part of the epithel um 

 of the wa'l of the oral cavity is 

 derived directly from the primitive 

 neural tube of the embry- (fig. 

 27), so that in fact the new mo^th 

 in these larvae is no hingbutihe 

 old mouth, though a shorter com- 

 munication with the archenteron 

 has been established. 1 hardly 

 n^ed emphasize once more, that 

 the circumstance that in the three 

 main groups of Chordates three 

 different mouths are found 

 strongly pleads for the supposi- 

 tion that the mouth of Chordates 

 is a secondary formation, a consequence of the loss of 

 the primary mouth, of which process my theory gives such 

 a simple explanation. 



Segmentation in Ascidians. — If now we try to account 

 for the further structure of the Ascidian larva and the 



mouth 



Fig 27. Diagrammatic 

 sagittal sections hrough 

 antero-dorsal part of As- 

 cidian embryos shcwi.ip 

 transfoimation of rejral 

 tube into wail of oral 

 siphon (after HuntSj-iAin, 

 1913). 



