452 TREAD WELL. [Vol. XV 1 1 . 



from ectoderm nor entoderm, but rather from cleavage cells, 

 inasmuch as at the time of formation there has been no differ- 

 entiation into germ layers, and he accepts Meyer's theory that 

 the mesoblast bands represent a gonad tissue, from which may 

 arise by differentiation all the mesoderm of the body, including 

 new germ cells. 



From these facts it seems to me, if we accept Professor 

 Wilson's theory, we are bound to believe in two non-homolo- 

 gous sets of larval mesenchyme ; the one arising from ecto- 

 derm, found, for example, in Podarke, and the other arising 

 from the anterior ends of the germ bands and found in Nereis 

 and Lumbricus. These two sets apparently do not, as a rule, 

 exist together, only one case, that of Capitella, having been 

 described. On the other hand, no one has proved, so far as 

 I know, that no " mesenchyme " arises from the germ bands 

 in cases where a larval mesenchyme exists. Either, then, 

 we must assume two sets of non-homologous organs having 

 exactly the same structure and function, and in at least one 

 case existing side by side, or (and this seems to me the more 

 reasonable assumption) we must regard both mesenchyme and 

 mesoderm as morphologically the same tissue, apparent differ- 

 ences in their mode of origin being of no significance. If, as 

 I believe, the trochophore represents an ancestral stage in the 

 phylogeny of the annelids, the mesodermal structures found in 

 it undoubtedly represent the mesoderm of the ancestral form. 

 Whether the origin of metamerism was due to a process of 

 budding, or to growth and subsequent metamerization, is not 

 essential for the present discussion. In either case, as Wilson 

 has pointed out (No. 34, c) (it is only fair to say that the fact 

 was used as an argument against the ancestral significance of 

 the trochophore), the trochophore at the present time is more 

 than a mere ancestral stage, for it contains in a concentrated 

 form the Anlage of the whole future body. Mead has proved 

 that in Amphitrite all the ectoderm for the body behind the 

 first septum arises from a group of cells which surround the 

 proctodaeum of the young trochophore and are descended from 

 a single cell, the first somatoblast, and probably the same thing 

 occurs in Podarke. The grouping of the mesoderm at the 



