MUSEUM OF COMPARATIVE ZOOLOGY, 69 



two cases. In Cristatella it is long, thick, and filled with a dense mass of 

 large cells (Figs. 95, cev. oce., and 101 *, 102*). In Plumatella (Fig. 99) 

 it is very short. 



The second difFei-ence concerns the embryo itself, and is connected 

 with the formation of the first polypide. In Plumatella (Fig. 99) the 

 first indication of the formation of the first polypide occurs at or 

 very near the neck of the ooecium, or, since the ingressiou of cells 

 into the blastocoel took place at the pole of the blastula nearest the 

 neck, we may say near to the pole at which ingression occurred. 

 The cells of the outer layer (^.) are elongated and contain large ellip- 

 soidal nuclei which are often pressed close together. All of the cells 

 of the larva stain more deeply at this pole than elsewhere, and those of 

 the inner layer rather more deeply than those of the outer. The nu- 

 clei are also very large, those of the outer layer being possibly more 

 prominent than those of the inner ; but the difference is not so marked as 

 in the drawing, where too the nucleoli of the inner layer are represented 

 relatively too small. Even at this stage one finds in another section 

 of the same embryo the beginning of a second polypide. whose position 

 is indicated at *. This second polypide is indicated merely by a con- 

 siderably thickened inner larval layer, and a very slightly thickened 

 outer one. The two polypides are thus seen to be wholly independent 

 of each othei'. The first invagination further advanced is seen in cross 

 section of the whole larva in Figure 96. The entire outer layer would 

 seem at first sight to be involved in this invagination ; but even in this 

 figure there are seen one or two nuclei which lie under the ooecium at 

 the place of invagination. I believe that they will not be involved in 

 it, for at a very little later stage (Fig. 104) one finds a layer of cells 

 lying over the invaginated bud, which I believe are destined to form the 

 ectoderm of the body wall at this place. 



Later stages in the development of the larva in this species are 

 not shown. The bud follows, I am confident, the same steps that are 

 pursued by the bud in the adult colony. A placenta-like connection 

 of the larva with the ooecium, which was first described by KorotnefF 

 ('87, p. 194), begins at about this stage, and continues until two well 

 formed polypides are present. This " giirtelformige Placenta" begins 

 to form in about the middle of the young embryo, and the elongated 

 cell of the outer layer of the larva, in contact with the ooecium shown 

 on the left of Figure 99 below the *, is, I believe, the first indication 

 of it. The ooecium and larva both continue to increase in size, and 

 the walls of the former become thinner with their increase in area. 



