1 72 WHITMAN. [Vol. I. 



the teloblasts may differ so little in size from the cells which 

 they produce that their terminal position is about the only 

 means of distinguishing them. This accounts for their having 

 been overlooked by such embryologists as Kowalevsky, Klein- 

 enberg, and Hatschek. In the Hirudinea we see the different 

 kinds of teloblasts of each band represented either by one or 

 two cells. Wilson informs me that in one species of Lumbricus 

 he finds only one nephroblast on each side; in another species, 

 two, as in Clepsine. 



We do not know to what extent this variation in number may 

 be carried ; but it adds another difficulty of recognition, which 

 might easily become insuperable. Instead of only one or two 

 teloblasts of a given kind, there may be many, all taking equal 

 shares in a common work, or correlative parts of a complex 

 work. Some such condition may be supposed to exist in tht 

 higher bilateral animals. 



We already have sufficient grounds for regarding the telo- 

 blasts as an archaic feature of development. Obviously they 

 do not represent primitive organs, but the undeveloped, embry- 

 ological bases of such organs. They constitute the trunk-bud, 

 and are thus the primary seat of all the truly metameric ele- 

 ments of the animal. Prin.arily they represented, as we have 

 reason to suppose, the bases of non-metameric organs, in which 

 the regenerative power was, or became, preeminent. 



6. TJie Fcetal a7id the Larval Type of Development. 



The relations of the foetal and the larval types of develop- 

 ment have never been made clear by those who hold that the 

 latter represents, approximately, the ancestral line of develop- 

 ment. Some have maintained that the phylogenetic history of 

 the annelid is retraced in larval metamorphoses ; while others 

 have denied any such morphogenetic significance to the larva, 

 claiming that it is a secondary form reached through adaptive 

 changes which have been called forth by its pelagic mode of life. 



Balfour has given us a broad and comprehensive discussion 

 of the nature, origin, and affinities of larval forms, and has con- 

 sidered, in a general way, the nature and extent of the second- 

 ary changes likely to occur in the foetal or the larval state. 

 According to Balfour (No. 32, p. 299), "the relative chances 



