DEVELOPMENT OF PLANULA IN A PLUMULARIAN HVDKOID. 95 



the covering-plate and of the distal endoderm of the blasto- 

 style become very attenuated and full of vacuoles, and 

 will subsequently disappear. The space (fig. 8, c. 0.) 

 between the egg and the ectoderm of the gonophore in- 

 creases in size, and the membrane around the ovupi (r. )ii) 

 is very pronounced. The operculum consists of thicker 

 perisarc than that of the rest of the gonotheca, and its 

 lower edge is sharply marked off from the thinner perisarc 

 below. The gonotheca has now assumed its definite form, 

 and it does not further increase in size ; its greatest width is 

 about 0"51 mm. and length 0"85 mm. 



At a somewhat later stage the outermost gonophore-layer 

 of flat ectoderm cells disappears and the iriner layer of 

 cubical or columnar cells breaks up to forfti an irregular 

 cluster around the ovum (fig. 9, e. G.). The covering plate 

 of ectoderm cells below the operculum and the endoderm 

 layer of the terminal portion of the blastostyle are still 

 faintly visible, but they are on the point of disappearing. 

 The out-pushing of endoderm {d. C.) or spadix of the 

 gonophore consists of cells which remain in an actively 

 living condition. 



The ovum does not grow, the protoplasm is finely granular, 

 and there is a large nucleus with nucleolus (fig. 9). There is 

 still a pronounced membrane around the egg ; but it is much 

 less developed on the inner side against the granular cells of 

 the endoderm than on the outer side. 



In the next stage it will be seen that considerable changes 

 have occurred in that the whole of the covering plate of 

 ectoderm and the distal horizontal portion of the endoderm 

 of the blastostyle have entirely disappeared. The out-pushing 

 of endoderm (fig. 10, d. C.) or spadix of the gonophore has 

 grown upwards, and has pushed the egg, together with the 

 surrounding ceils derived from the breaking-up of the 

 columnar ectoderm cells of the gonophore, into a more or less 

 central position in close contact with the operculum. The 

 vitelline membrane or mesogloea-layer round the ovum be- 

 comes less distinct. The cells around the ovum arrange 



