432 WILSON-. [Vol. VI. 



is shown in Diagram V, C, where the neural and mesoblast axes 

 practically coincide. It is probable that the actual space be- 

 tween the residual teloblasts and the pigment-area is never 

 obliterated — i.e. that the rotation is completed simply by the 

 form of growth as the body elongates. I may point out that 

 in Lumbricns also the posterior extremities of the neural and 

 mesoblastic bands are separated by a considerable space. This 

 shows simply that the posterior extremity is not a geometrical 

 point. 



We may finally review in a few words the axial history of the 

 upper hemisphere. The facts stated at p. 425 in regard to the 

 frontal bodies and the cerebral ganglia render it nearly certain 

 that the centre of the upper hemisphere lies in the dorsal sur- 

 face and does not represent the anterior extremity. The polar 

 cells are extruded at this point, where they persist up to a late 

 stage {Fig. 41). There is no doubt that this point coincides 

 with the centre of the cross, and von Wistinghausen's observa- 

 tions on N. Ditvierilii, in connection with my own, leave little 

 doubt that the centre of the cross coincides with the centre of 

 the cephalic neural plate, from which the cerebral ganglia de- 

 velop. These ganglia, however, lie behind the anterior extrem- 

 ity (p. 425, and Diagram V, C), the position of which is nearly 

 indicated by the position of the frontal bodies. The anterior 

 and posterior extremities are, however, but vaguely defined in 

 the early larva. They, like all the other features of the body, 

 are only gradually marked out as the development goes forward. 



IX. Comparison with Other Forms. 

 I. Comparison with Other Species of Nereis. 



The works of Gotte and Salensky, which have already been 

 briefly reviewed at p. 369, are too fragmentary to afford a satis- 

 factory basis for a comparison of the early stages with the 

 American species, and I shall therefore depend mainly upon v. 

 Wistinghausen. Still, both the former authors observed certain 

 important points which it is desirable to consider. 



Gotte {N. Dumerilii, free-swimming trochophore) figures cor- 

 rectly the first three cleavages, and gives also the true orienta- 

 tion of the embryo. He then describes accurately the formation 



