40 BIOLOGICAL LECTURES. 



retained but adapted to a new use, precisely as has been the case 

 with the evolution of larval or adult organs, such as the branchial 

 or aortic arches and the limbs. As the change progressed the 

 posterior cell of the fourth quartet became more and more 

 strictly given over to the formation of mesoblast, its entoblastic 

 elements becoming correspondingly reduced to truly rudiment- 

 ary or vestigial cells {Aricia, etc.), or finally, perhaps, disap- 

 pearing wholly. 



I have endeavored to place these special conclusions in 

 strong relief, not because they can yet be accepted as demon- 

 strated, — and it is quite possible that some other interpreta- 

 tion may yet be placed upon some of the facts, — but because 

 they seem to me highly suggestive of further research in the 

 field of cell-lineage. There are among them two general con- 

 siderations on which I would lay emphasis. 



First, the study of cleavage or cell-lineage in the case of 

 these groups raises a number of highly interesting and sug- 

 gestive questions in pure rnorphology. If the mesoblast-bands 

 are a new formation, what is the motive, so to speak, for their 

 origin.' Did they perhaps arise through the development of a 

 new body-region, or a new growth-zone, or budding-region from 

 the posterior part of the ancestral body, as has been assumed 

 by Leuckart, Haeckel, Hatschek, and Whitman in explanation 

 of metamerism.'' Is the body of the turbellarian homologous 

 to the entire body of an annelid or mollusk, or does it repre- 

 sent only the head or the larval body, to which a trunk-region 

 is afterwards added.'' What is the relation of the entomeso- 

 blast to the archenteric pouches of the enterocoelous types.'' 

 How do the above fesults harmonize with the general doctrine 

 of development by substitution.' These are examples of some 

 of the morphological questions suggested by the general in- 

 quiry. They are admittedly of a highly speculative character, 

 and I, for one, am not prepared to give a positive answer to 

 any of them. But the mere fact that morphological questions 

 of such character and scope are inevitably suggested by studies 

 in pure cell-lineage shows that such studies must not be passed 

 over by the morphologist as having no interest or value for his 

 own researches. 



