TROPIOMETRA CARINATA. 



11 



3. VEST1BULARY INVAGINATION ; FURTHER SPECIALIZATION OF THE 

 OELOM AND HYDROCCEL, 16 TO 40 HOURS. 



In embryos 16 hours old the ciliated bands are fully formed. There are 

 only 4 of them, not 5 as in Antedon adriatica and mediterranea (plate x, 

 figures 1 and 2). Wyville Thomson figures only 4 ciliated bands in the 

 species which he has studied, Antedon bifida, so that it would appear that 

 even within the same genus the number of the ciliated bands may be variable. 

 However, this certainly needs confirmation, as it seems doubtful that so 

 conspicuous a difference would exist in closely related species. 



The second band apparently ends abruptly on the sides of the vestibulary 

 invagination, but on closer observation it may be seen to continue along the 

 borders within the vestibulum, around its posterior end, although in the 

 specimen figured in plate x, figure 2, this could not be distinctly ascertained. 

 The first band is pushed slightly upwards, the third slightly downwards, by 

 the vestibulary invagination. The posterior ciliated band lies in a slight 

 depression, as is seen distinctly in most of the sections (e. g., plate in, 

 figures 4 and 6). The anterior or apical pit is not so distinctly circum- 

 scribed, as in Antedon. 



The vestibulum, which begins as a flattening of the ventral side of the 

 embryo at the age of 12 hours, now forms a distinct invagination of broad 

 oval outline. 



The ectoderm is distinctly limited towards the mesenchyme in embryos 

 16 to 20 hours old, but from the age of 25 hours no limit can be seen. In 

 sections stained with hematoxylin, elements are seen in the ectoderm which 

 stain very strongly and look like glandular cells (plate v, figures 1 to 5, 

 gl. c.)- Although the histological preservation is not quite satisfactory, I 

 have no doubt that. these elements correspond to the "yellow cells" of the 

 Antedon larva, which are also supposed to be of glandular nature. In the 

 vestibulary invagination the ectoderm stains very strongly in hematoxylin, 

 a feature which does not depend alone on the fact that the nuclei are here 

 much more numerous than in the other parts of the ectoderm (plate iv, 

 figures 3 and 13; plate v, figure 6). Possibly this indicates a glandular 

 character of the cells of the invagination. Bury (p. 269, plate 44, figure 22) 

 mentions the same feature in Antedon, but points out that they lose their 

 color much more readily in acidulated alcohol than do the glandular cells 

 of the ectoderm (c/. also Seeliger, p. 246). Bury finds these deeply staining 

 cells only in the anterior, deeper part of the vestibulary invagination. In the 

 Tropiometra larva the cells have this character throughout the whole length 

 of the invagination (plate iv, figure 13). 



In a specimen 25 hours old (plate iv, figure 2) I have observed very 

 distinctly a feature described by Seeliger in Antedon (p. 236), viz, that the 

 ectoderm cells secrete between themselves an intercellular substance, this 

 process being the beginning of the transformation of the ectoderm, which 



