428 WILSON. [Vol. VI. 



{X) are the homologues of the neuro-nephroblasts of the Hiru- 

 dinea and Oligochseta, which lie at the posterior extremity of 

 the germ-bands. The position of these teloblasts in Nereis 

 must, therefore, also mark the posterior extremity ; yet they 

 lie in contact with the prototroch, ninety degrees away from the 

 centre of the pigment-area. 



This curious discrepancy appears at first sight to be a fatal blow 

 for the " Achsenzoologen." It appears to me, however, simply 

 to demonstrate the truth of the view urged by Hatschek and 

 others that a shifting of the axes takes place in the course of 

 the metamorphosis. The discrepancy is due in Nereis simply to 

 the fact that the mesoblast axis and the neural axis do not shift 

 at the same time. The former takes up its definitive position 

 much earlier than the latter, so that the larva passes through a 

 transitional period in which the two axes form an angle with 

 each other {E). I think it will be easy to show that tJie precocious 

 shifting of the mesoblast-axis is caused by the early concentration 

 of the material of the ventral plate in the first somatoblast. To 

 establish this proposition will require some consideration of the 

 gastrulation and trochophore-formation in other annelids. 



It may, I think, be taken as a well-established fact that 

 throughout the trochopore series (Annelida, Mollusca) the blas- 

 topore occupies primarily the central portion of the lower hemi- 

 sphere (" Gegenfeld "), and its margins are parallel to the plane 

 of the prototroch, which is often well developed long before 

 the close of gastrulation. The mesoblast-bands arise from cells 

 (usually, perhaps in the last analysis always, a single pair of telo- 

 blasts) that lie symmetrically in a definite position at the lip of 

 the blastopore (Diagram VI, A). These cells define not only the 

 median plane, but also the posterior region of the embryo, and 

 by almost all writers this portion of the blastopore-margin is 

 designated as the posterior lip. The closure of the blastopore 

 typically proceeds forwards from the posterior lip, in the direc- 

 tion of the arrow, its anterior portion usually persisting as the 

 mouth, or marking its position. During, or sometimes before, 

 the process of closure the mesoblast-cells pass into the cleavage- 

 cavity, where they ultimately give rise to the mesoblast-bands. 



Among the annelids the axial relations of the mesoblast-bands 

 (and also of the neural bands) with reference to the blastopore 

 are of two types, (i) In the first type, typically 7'epresented by 



