58 



S. H ATTA. THE FATE OF THE PERISTOMA!, 



folded to give rise to the median fin, but retains its connection with the tail 

 bud, from which it is sharply marked off by the protoplasm of its com- 

 ponent cells less stained than that of the cells composing the latter. 



The indifferent elements in the tip of the tail, which constitute the tail 

 but, are differentiated into the chorda and the caudal somites. The cell 

 division, by which the cells of the tail bud are multiplied, is by stages, 

 diminished in its activity. Accordingly the somites and chorda formed out 



of them are diminished by stages, 

 i.e. from before backwards, in thick- 

 ness. In a larva about thirty days 

 old, in which the postanal gut has 

 a long time ago been altogether 

 degenerated (see below), the tail bud 

 attains almost its limit of growth ; for 

 in a larva a little further advanced, 

 no trace of the tissue constituting 

 the organ can any longer be detected. 



It is, of course, not easy to say 

 whether the last remnant of the tail 

 bud (Photogram 3) represents the 

 hindermost, unpaired somite or the 

 indifferent tissue which is capable of 

 being- still further differentiated into 

 the chorda and the somites. But 

 there are facts which account for 

 favour of the assumption that it is 

 the mesoderm which constitutes the 

 hindermost somite. In the first place, 

 the chorda grows no longer by its 

 distal extremity, so that the tissue in 

 question is employed no more in the formation of the chorda. In the 

 second place, the indifferent tissue is altogether resolved into free 



Photogram 3: Frontal section through tail 

 of a larva 14 days old. 



c. chorda, c. ectoderm, f. fin-fold. 

 g. gut. ?'. indifferent cell structure of tail 

 fold. m. mesoderm, v. neural canal, 

 mesodermic somite. 



