ORIGIN AND STRUCTURE OF THE HEAD 135 



stigmata in the branchial sac of the former from three pairs 

 of primary gill-clefts. The question whether these gill-slits 

 by their situation indicate perhaps the limits of still more 

 original trunk segments must be left open. 



It needs hardly be repeated once more that I cannot agree 

 with VAN Wyhe's (1893, p. 155) opinion that the mouth of 

 Ascidians is to be fourd again in Amphioxus as the left 

 anterior intestinal diverticulum and that its opening would 

 represent ihe former mouth ("autostoma ') of Amphioxus; 

 neither dees VAN WYHE's comparison of ihe anus of Ascidian 

 larvae wth the gill-slit of the left side following behind 

 the mouth in Amphioxus seem to me very acceptable. 1 feel 

 more inclired to support WILLEY (1893, p 349, in his view 

 that the U shaped alimentary caral is a consequence of the 

 sessile habit of life, which may even have been the original 

 condition of Afpendiculaiia also. 



A question, the discussion of which I will also leave to 

 those who make a special study of these forms is, whether 

 the tail of Ascidian larvae and Appendicularia corresponds 

 to the tail of Vertebrates generally (WiLLEY, 1893, p. 346) 

 or whether it comprises part of the trunk of the latter 

 (VAN Beneden and JULIN, 1887, p. 387). 



History of the Vertebrate head. — We shall finish this 

 chapter with an at'empt to give a short summary of the 

 history of the Vertebrate head, drawn from the results of 

 the numerous authors who have worked on the subject. 

 These results, however, will be arranged and combined 

 after the principles put forward in the preceding pages. 



Animal pole of egg and blastula. — The history of the 

 Vertebrate head is closely connected with that of the animal 

 pole of the egg and of the blastula. The latter stage 

 of development, through which all Metazoa pass, was termed 

 by Huxley (1877, p. 678) the "animal Volvox" and, indeed, 

 in tracing the history of the a.imal pole. 1 believe we must 

 go back to this colony of Flagellates which may be con- 

 sidered with equal right to be a lowly organized individual 

 (BuTSCHLI, 1883, p. 775). As put forward by JANET (1912) 

 in an interesting article to which I refer for the literature 

 relating to this subject, it represents by no means a 

 homaxonous sphere, for a main axis can indeed be 

 distinguished. The shape of the colony is not spherical 

 but somewhat elongated in the direction of the main axis 

 round which the colony rotates and which indicates the 



