36o DISCOVERY REPORTS 



was not possible, however, to study the histology in detail, this being reserved for a 

 future occasion. The illustrations given represent optical sections and are intended 

 mainly as accessory data for systematic study. 



The embryonic development in T. charcoti, in which species a nearly complete series 

 of embryos was obtained, may be conveniently dealt with under three headings : (i) the 

 segmentation of the ovum and the differentiation of the cells so formed, (ii) nutrition, 

 and (iii) the development of the spicules. 



Tedania charcoti, Topsent 



Segmentation. The ovum is first seen, in the deeper parts of the choanosome, as a 

 semi-transparent body, charged with refringent granules, oval in shape, o-i by 0-07 mm., 

 and lying in a capsule which is itself connected with the choanosomal tissues by radiating 

 suspensoria (Fig. 47 a). The nucleus is at first faintly visible, but with the accumulation 

 of food material is rapidly obscured. The first cleavage results in two cells of approxi- 

 mately equal size (Fig. 47 e), but the second gives rise to four cells of appreciably 

 different sizes (Fig. 47/). From this point onwards segmentation becomes very irregular 

 (Fig. 47 li), but results finally in a spherical mass of cells (Fig. 47 i) of approximately 

 equal size. Details of the intervening stages are missing, and the next stage in develop- 

 ment seen is that represented by Fig. 47 y, in which the embryo, now oval and measuring 

 0-24 mm. in longest diameter, consists of a dense central mass of small granular cells, 

 measuring 0-014 mm. in diameter, covered externally by a thin layer of much smaller, 

 flattened cells. It is at this point that the first spicules make their appearance. After this 

 the embryo increases considerably in size, becomes approximately spherical, 0-35 by 

 0-32 mm., and nearly transparent. A cortical layer of columnar cells becomes visible, 

 0-035 mm. thick, and the cells of the inner mass, now slightly smaller in size, are much 

 more diffusely distributed. The succeeding stages are concerned mainly with changes 

 in the distribution of the spicules (Fig. 47 /), and finally, a change in the outline of the 

 embryo itself (Fig. 47 m). This latter is probably concerned solely with the escape of the 

 embryo from the capsule, the walls of which have grown very thin (Fig. 47 k), and it 

 appears as if the embryos are liberated from the capsule and make their way to the 

 exterior by a series of quasi-euglenoid movements. 



Nutrition. In the earliest stages, the embryonic capsule is thick-walled, and 

 entirely empty except for the ovum, but in its walls may be seen a number of granular 

 cells, spherical or oval in form, and measuring, on an average, o-oi mm. across. Similar 

 cells may also be seen in the suspensoria and in the adjacent choanosomal tissues. There 

 can be little doubt that these are nutritive cells. Immediately prior to the first cleavage 

 (Fig. 47 b) they become much more numerous and a few are seen migrating into the 

 capsular cavity (Fig. 47 c), and as segmentation proceeds the cavity becomes partially 

 filled with a granular mass (Fig. 47 d), derived presumably from the breaking down 

 of these cells. The ovum, too, becomes more opaque and granular and assumes a 

 golden yellow tint. After the second cleavage, the nutritive cells become appreciably 

 less in the capsular wall (Fig. 47/), the colour of the embryo deepening slightly, until 



