165 



chiefly on the metamerism which is supposed to be derived from the 

 pouchiog of the actinian gut by the mesenteries — and on the slit- 

 like appearance of the deuterogenetic centre in those embryos most 

 distorted by a great accumulation of yolk. 



It is difficult to have much faith in the primitive groove — as 

 groove — representing the aperture of the blastopore. We are bound 

 to take into consideration the mechanics of development, and there 

 are many features in the formation and growth in form of the primi- 

 tive streak and groove which most certainly are open to a mechanical 

 explanation (Assheton, 1). Nor again must one forget that the neur- 

 enteric canal is not the same thing as a blastopore. The blastopore be- 

 longs to the protogenetic epoch, the neurenteric canal to the deutero- 

 genetic epoch. 



Sedgwick and Hubrecht take it for granted that metamerism 

 must have been laid down in the radially symmetrical animal. Is this 

 supported by embryological evidence? I think not. To take two cases, 

 a vertebrate (Frog) and Annelid (Polygordius). There is no sign of 

 metamerism in the radially symmetrical protogenetic part of the em- 

 bryo. This has been commented on by Jablonowski (11). All the 

 metamerism which in both cases is very well marked, occurs in the 

 deuterogenetic region. It is intimately associated with the latter. 



Bateson (4) in discussing the repetition of parts deprecates 

 the assumption that metamerism in two groups of animals necessarily 

 indicates close relationship. He writes, p. 20: "The first difficulty 

 which has been brought into morphology by the suggestion that Meta- 

 metric Segmentation is a phenomenon distinct in kind, is one which 

 has coloured nearly all reasoning from the facts of Morphology to 

 problems of phylogeny. For the existence of Metameric Segmentation 

 in any given form is thus taken to be one of its chief characters, and, 

 as such, is allowed predominant weight in considering the genetic re- 

 lation of these forms All alike" (i. e., certain theories of 



phylogeny) "are founded on the assumption that resemblances between 

 the manner and degree in which Repetition occurs are unlikely to 

 have arisen save by community of Descent. A broader view of Me- 

 ristic phenomena will shew that this assumption is unfounded; for so 

 far are the expressions of it which are called metamerism from standing 

 alone, that it is almost impossible to look at any animal or vegetable 

 form without meeting phenomena of Repetition which differ from Meta- 

 merism only in degree or in extent." 



Growth in length by the addition of new material to pre-existing 

 organs from a terminal growing point as in the Vertebrate (or nearly 



