BLASTOPORE, MESODERM AND METAMERIO SEGMENTATION. 21 
I. The Obliteration of the Relation of Blastopore 
to Mouth and Anus. 
Previous Observations. — Since the time of the Gastraea 
Theory many writers have occupied themselves with the blas- 
topore. Lankester (This Journal, 1877), in accordance with his 
planular theory, came to the conclusion that the coincidence of 
mouth and anus with the blastopore was only a developmental 
convenience. He says (loc. cit.), “ Regarding, as I do, the blas- 
topore as an orifice of a secondary nature, existing solely in rela- 
tion to the invagination process, and originating after mouth 
and anus had made their appearance in the progress of animal 
evolution, I seek to explain its occasional relation to the mouth 
and to the anus as cases of adaptation.” Balfour, after enu- 
merating the different fates of the blastopore in the animal 
kingdom, says, “ It is clearly out of the question to explain all 
these differences as having connection with the characters of 
ancestral forms. Many of them can only be accounted for as 
secondary adaptations for the convenience of development.” 
The number of groups in which a slit-like blastopore has been 
described is very considerable (vide Balfour, vol ii, p. 282). 
Lankester was the first to suggest that a slit-like blastopore 
which might close at either end would, if taken as the ancestral 
type, account for the various fates of the blastopore in mol- 
luscs. Hatschek, in his paper on Teredo, has suggested the 
possibility of phylogenetically deriving the anus which arises 
in this animal ; secondarily, as an ectodermic invagination from 
part of a slit-like blastopore. But he bases this view on the fact 
that the anus corresponds in position with the hind wall of the 
gastrula mouth. Metschnikoff, in combating Hatschek’s views 
on the early expression of bilateral symmetry, has denied the 
ancestral character of the slit-like blastopore. He says, “ Kann 
man dem geschlitzten grossen Blastopor keine palingenetische 
Bedeutung zuschreiben und muss ihn als eine embryonale 
Anpassungs-erscheinung ansehen.” Sedgwick (loc. cit., p. 27) 
concludes that “ the mouth and anus of the Triploblastica are 
