EMBRYONIC DEVELOPMENT 361 



in the later stages of segmentation they are only sparsely present. At this stage, raphides 

 from the maternal tissues are carried in to support the wall of the capsule, which is from 

 then onwards progressively reduced in thickness (Figs. 47 h-j). Finally, the wall loses 

 its layer of raphides and becomes extremely thin (Fig. 47 k) in preparation for the escape 

 of the embryo. The appearance of the raphides in the capsular wall appears to mark 

 definitely the end of the nutritive period. 



Development of Spicules. The first spicules to appear are extremely fine acan- 

 thostyU (Figs. 477 and 53 a), 0-087 by 0-003 mm., lying in (or immediately beneath?) the 

 outer layer of cells (cf. also T. teniiicapitata, Fig. 48 c). With the further diff'erentiation 

 of the embryonic tissues, as, for example, the formation of the outer cortical layer, the 

 spicules migrate inwards, and become converted into smooth styli, measuring 0-14 by 

 0-007 mm. At the same time, small raphides, 0-07 mm. long, appear, scattered generally 

 throughout the inner cell mass (Fig. 47 k). The further growth of the embryo is marked 

 by a segregation of the styli in a neat bundle at the aboral pole of the embryo and a rapid 

 multiplication in the numbers of the raphides (Fig. 47 / and m). 



The raphides, the prototypes apparently of the adult onychaeta, resemble acanthostyli 

 in their early stages, and it is probable that they are modifications of this spicule form, 

 and not modifications of oxea, as is often inferred, and as appears to be the case in the 

 raphides of other groups. 



T. tenuicapitata, Ridley 

 Only a few embryos were found in three specimens, but from these it is clear that the 

 development follows the same lines as that of T. charcoti. The earliest stage observed 

 consists of a spherical mass of polyhedral cells contained in a capsule, the walls of which 

 are strengthened with small onychaeta and contain a few nutritive cells (Fig. 48 a). 

 The next stage found (Fig. 48 b) is of interest because in this the formation of the outer 

 unicellular layer is seen to be in progress, but has not advanced sufficiently far for the 

 inner cell mass to be completely covered. In addition, the formation of spicules can be 

 seen to have begun in those parts of the outer layer which are well established. Fig 48 c 

 represents a section through an embryo at a slightly later stage, and is intended to show 

 the approximate position of the spicules in the early stages of their development. Fig. 48 d 

 represents the latest stage obtained. The first spicules to appear are small roughened styli, 

 o-i by 0-004 mm., followed by raphides, 0-07 mm. long. 



T. nmrdochi, Topsent 

 Only the later stages were observed ; and although the histology appears to be similar 

 to that of T. charcoti, there are several points in which the embryos of the two species 

 difi'er. The earliest embryo of T. nmrdochi is spherical and measures 0-28 mm., and 

 the most remarkable feature about it is the manner in which the surface is thrown 

 into loose folds (Fig. 50 a, b). The spicules are roughened subtylostyli, 0-14 by 0-003 mm., 

 and raphides, o-i mm. long, and it may be remarked that, in contradistinction to the 

 spicules of the embryos of other species, the subtylostyli of T. nmrdochi are arranged in 

 opposite directions, some with the base directed orally and some with the base directed 



