2l6 WILSON. [Vol. IV. 



wedged into the angle between the posterior part of the archen- 

 teron and the ectoblast : no one of them can be identified as the 

 teloblast. Besides this " secondary mesoblast " there are at this 

 period a number of scattered cells, lying in the narrow cleavage- 

 space. Some of these are applied to the ectoblast, some to the 

 entoblast, where they form a contractile network surrounding 

 the archenteron ; some stretch across the cleavage-cavity. These 

 cells obviously represent a part of the " primary mesoblast " or 

 "mesenchyme," the relation of which to the secondary meso- 

 blast of the germ-bands has become one of the most interesting 

 questions of annelid embryology. My observations on Hydroides 

 indicate that the primary mesoblast arises in a manner similar 

 to the process in Polygordms, as described by Repiachoff^ 

 and Metschnikoff,^ though I am not yet able to give decisive 

 evidence, in spite of repeated examination of the question. 

 According to these authors the primary mesoblast arises in the 

 form of isolated cells that take their origin "hochst wahrschein- 

 lich," in the entoblast, at or just before the time of gastrulation. 

 Metschnikoff makes no special mention of the formation of the 

 secondary mesoblast-bands ; but his general account implies, 

 and his figures bear out this interpretation as far as they go, 

 that there is no real distinction between the primary and sec- 

 ondary mesoblast, the latter arising in exactly the same manner 

 as the former. For the present I am obliged to believe that 

 the primary and secondary mesoblast have the same relation 

 in Hydroides. The mesenchyme cells are of all shapes, — ■ 

 branching, elongate, rounded, — and appear to graduate both 

 in form and in position into those of the germ-bands. The 

 latter appear about the time of gastrulation, as two bilateral 

 masses of cells that are pushed into the cleavage-cavity near 

 the blastopore. Some of these cells appear to pass forwards 

 and give rise to the "mesenchyme"; the remainder form the 

 " secondary " mesoblast-bands, which at first are posteriorly at 

 the sides of the proctodseum, but afterwards come into con- 

 nection by a bridge that is developed on the ventral (anterior) 

 side of that structure. 



Through the courtesy of Mr. Agassiz I have had an oppor- 

 tunity to investigate Polygordius, the larvae of which are very 



^ Zool. Anzeiger, i88l. 



- Z.f. W. Zool, XXXVII., 1882. 



