286 Transactions. 



Art. XXII. — Preliminary Note on some Stages in the Develop- 

 ment of a Polychaete. 



By H. B. Kirk, M.A., Professor of Biology in Victoria College, 



Wellington. 



Plate XXIII. 



In January, 1906, I obtained at Plimmerton some very interest- 

 ing egg-masses of a Polychcete. The facilities for examination 

 were not great ; but I was able to keep the developing larvae 

 under observation for five days, although not without inter- 

 mission. This year I failed in several attempts to obtain suit- 

 able material for further observation. 



The masses were found cast up by the tide. In texture they 

 are gelatinous, and in shape they somewhat resemble a barrel 

 open at both ends. The length of the masses is from 20 mm. to 

 25 mm. The shape of any one of them leaves no doubt that it 

 was produced by a worm provided with a cingulum. 



In the gelatinous matter are imbedded eggs and larvae, the 

 latter in various stages of development, owing probably to 

 fertilisation having been effected at different times. Each 

 ovum has a thin membranous envelope, and, outside this, an 

 envelope, possibly albuminous, more highly refractile than the 

 surrounding gelatinous matter. 



Embryos in the earlier stages of development were few. 

 The earliest stage of which I could be certain was one in which 

 eight megameres were surrounded by micromeres, apparently 

 sixty-four in number, but of that number I could not be quite 

 sure. 



No trochosphere stage was observed with any certainty. 



The earliest certain indication of segmentation of the body 

 is in the appearance of a slight transverse constriction in its 

 anterior part, the body being now broadly oval in shape. This 

 constriction marks the division between the peristomium and 

 the next following segment. 



The appearance of the first pair of parapodia was not noted. 

 Stages with the chaetigerous sacs of three segments well formed 

 were abundant, and in many of the specimens none of the chaetae 

 had yet reached the surface. In this stage ccelomic pouches 

 are observable ; but these correspond with the segments only 

 on the left side. On the right side one pouch is often large ; 



