69 
stigmata in the branchial sac of the former from three pairs 
of primary gill-clefts. The question whetker 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 1 cannot agree 
with VAN WYHE's (1893, p. 155) opinion that the mouth of 
Ascidians is to be found again in Amphioxus as the left 
anterior intestinal diverticulum and that its opening would 
represent the former mouth (“autostoma”’) of Amphioxus; 
neither does VAN WYHE's comparison of the anus of Ascidian 
larvae with the gill-slit of the left side following behind 
the mouth in Amphioxus seem to me very acceptable. l feel 
more inclined to support WILLEY (1893, p. 349) in his view 
that the U-shaped alimentary canal is a consequence of the 
sessile habit of life, which may even have been the original 
condition of Appendicularia also. 
A question, the discussion of which 1 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 attempt to give a short summary of the 
history of the Vertebrate head, drawn from the results of 
e 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 arimal pole, Il believe we must 
go back to this cofony of Flagellates which may be con- 
sidered with equal right to be a lowly organized individual 
(BürscHL1 1883, p. 775). As put forward by JANET (1912) 
in an interesting article to which 1 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 
