392 WILSON. [VoL. III. 
entoblast, which alone are of fundamental significance. With- 
out presuming to question the justness of these conclusions, 
which are certain to exercise an important influence on embry- 
ology, I wish to point out the fact that Kleinenberg’s views 
appear to have been influenced in a measure by his conclusions 
in respect to the origin of the “mesoblast” in Lumbricus, and 
that these conclusions are certainly erroneous. In his paper on 
the development of the earthworm Kleinenberg obviously means 
that the layer of cells ordinarily called “mesoblast” in this 
animal —that is, the layer of undifferentiated cells from which 
the muscles, blood-vessels, dissepiments, peritoneal epithelium, 
and reproductive elements arise —has a double origin, part 
arising from the primary mesoblasts at the posterior ends of 
the germ-bands, but a greater part being directly derived by 
proliferation of the overlying ectoblast. But after the most 
careful study of the question by means of complete series of 
sections of all stages of development, I can assert positively that 
this is not the case. It is true that new elements are added to 
the germ-bands from the ectoblast; it is not true that these 
elements assume the character of mesoblastic cells and give 
rise to structures ordinarily recognized as mesoblastic. On the © 
contrary, they are from first to last ectoblastic structures in the 
sense of being derived from the outer layer of the gastrula long 
after the layers are completely established, in retaining the 
histological characteristics of the ectoblast, in remaining long 
embedded in that layer, and in giving rise to structures (nervous 
system, setigerous glands ) that in other forms are unquestionably 
of ectoblastic origin ; and it would lead to hopeless confusion in 
the use of terms to regard the middle stratum as forming a part 
of the “ mesoblast,” taking the word in the sense employed by 
Kleinenberg in his paper on Lumbricus. It is furthermore 
perfectly clear that the middle stratum of Lwméricus in no way 
represents that part of the “ mesoblast’”’ (using the word in the 
ordinary sense) that is split off from the ectoblast in Lopadorhyn- 
chus (e.g. the muscle-plates ), for there can be no question that 
the muscles, dissepiments, vessels, peritoneal epithelium, repro- 
ductive organs, and a part of the nephridia, are derived from the 
inner or mesoblastic stratum, and hence ultimately from the two 
primary mesoblasts. 
This preliminary explanation will, I trust, make plain in what 
