No. 482] M ERISTIC HOMOLOGIES IN VERTEBRATES 105 



the pharynx and the Hver. Is this to be explained by saying that 

 in the vertebrates the posterior gill clefts of Amphioxus have closed 

 and that the space which they occupy has become converted into 

 stomach and oesophagus? In other words are these formations 

 of the vertebrate tube the homologues of a part of the gill region 

 of the acraniate ? 



Then, too, what are to be done with cases of increased numbers 

 of gill slits; the Notidanids with six or seven, the Californian 

 Bdellostoma with its variable number, and Amphioxus itself? 

 This question is wholly apart from that which discusses the rela- 

 tions between metamerism and branchiomerism. 



Numerous other similar questions will readily suggest them- 

 selves to all. There is no reason for enumerating them here. 

 The problem is, how are they to be explained. Must we find a 

 separate explanation for each or can we find some one principle 

 which will account for all? 



This article is to be regarded in the light of a suggestion rather 

 than a full reply with demonstrations of validity. I have no 

 proof, other than analogies and the fact that the hypothesis here 

 presented answers all the demands of the problem, that the expla- 

 nation here advanced is the true one. It must be tested and the 

 tests are not easily made. 



In the invertebrate segmented animals there is, at the beginning, 

 no metamerism. It appears later during growth, and in numbers 

 of forms it is found that the segmenting tissues are protkiced by 

 budding from groups of cells at the posterior end of the eiiihiyo. 

 These are most familiar in the annelid teloblast and are sr arccly 

 less well known in the Insects and Crustacea. Their number 

 varies between wide limits, but for the present purposes the most 

 important points concerning them, aside from their budding 

 capacities, are their position in a more or less plainly marked 

 transverse band and their situation at the extreme posterior limit 

 of the growing embryo. Extensive examination of the literature 

 has not shown similar budding cells in the Cuvierian group of 

 Articulata in other places than the tip of the growing embryo, 

 with the exceptions noted below. 



It follows then that in these teloblasts and their equivalents are 

 the full potentialities of the future somites. From them arise 



