126 MORPHOLOGICAL DEVELOPMENT. 



partially segmented, in the lowest Annelid. Again, while it 

 is said that the gill-clefts exhibit segmentation, it is admitted 

 that this has no relevance to any constitutional segmenta- 

 tion : " they are segmented on a plan of their own " irre- 

 spective of other organs. Another allegation is that the 

 ovaries of Amphioxus are segmented. Their segmentation, 

 however, like that of the gills, is isolated, and may be con- 

 sidered as illustrating those repetitions of like parts seen in 

 supernumerary vertebrae in various creatures a repetition 

 which becomes habitual if the resulting structure is advan- 

 tageous to the species. On the statement that while the 

 Amphioxus has no rudiments of a renal system the Elasmo- 

 branch embryo has such rudiments, which are as distinctly 

 segmented as the nephridia of a worm, two comments may 

 be made. The first is that if in these Vertebrates the 

 nephridia bear a relation to the general structure like that 

 which they do in Annelids, then one would expect to find 

 the segmental arrangement shown in the lowest type, as in 

 Annelids, rather than in a type considerably advanced in 

 development. Should it be replied that in the Amphioxus 

 an excretory system had not yet arisen, though one is re- 

 quired for the higher organization of an Elasmobranch, then 

 the answer may be that since the segmental arrangement in 

 the Elasmobranch corresponds with that of the myotomes, it 

 has no reference to any primordial segmentation, since the 

 myotomes have been functionally generated. The second 

 comment is that whereas the nephridia of the Annelid have 

 independent external openings, the nephridia in the Elasmo- 

 branch have not. These discharge their secretions into cer- 

 tain general tubes of exit common to them all; showing that 

 each of them, instead of being a member of a partially inde- 

 pendent structure, is united with others in subordination to 

 a general structure. That is to say, the segmentations are 

 far from being parallel in their essential natures. The asser- 

 tion accompanying these criticisms, that there is " no differ- 

 ence in principle between the segmentation of Amphioxus 



