29 



this contimies the segmentation cavity becomes so much decreased 

 that further growth must take place outwards, causing the subse- 

 quent evagination which leads to the stretching and rupture of 

 the gastral epithelium (Fig. 11). Previous to the setting free of 

 the embryo, the columnar cells have developed cilia, but having 

 examined onlj^ preserved material, I have, of course, not observed 

 these. 



Up to the time of writing I have had no opportunity of follow- 

 ing the post-embryonic life of this sponge. A couple of specimens, 

 measurmg 1-5 mm. were sectioned transversely and showed a ring 

 of oval flagellate chambers surrounding a central cavity, and 

 having distinct inhalent and exhalent canals showing that even 

 such minute specimens as these have attained to the adult form. 



Orientation. — The question of the orientation of the sponge 

 embrj^o is one which seems to have received little attention, though 

 the apparent lack of attention may indicate rather a lack of definite 

 results on this point. There is httle difficulty in determining the 

 polar bodies in connection with the ova, but to detect them ^Yith. 

 certainty in later stages when they have more or less degenerated 

 and He amongst numbers of small choanocytes is by no means an 

 easy matter. From the few doubtful cases I have seen, however, 

 it would appear that these bodies lie at the -pole occupied by the 

 columnar {i.e., future ciliated) cells. {See Fig. 13.) This view 

 is supported by a small piece of negative evidence. The 

 proximity of the choanocj^tes makes the polar bodies difficult to 

 identify, and in the majority of cases at least — according to 

 Dendy, in every case — it is the columnar cells w^hich lie next to 

 these so that the granular pole of the embrj^o is less frequently 

 hampered in this respect. The position of the embryo, then, is 

 such that, did the polar bodies he at the granular pole, they would 

 be comparatively easy to identify. Then' apparent absence then 

 from this pole, together ^^ith a few^ doubtful cases of their presence 

 at the opposite pole would seem to suggest that the columnar, 

 cihated cells are developed at the animal pole, which is the anterior 

 end of the free-swimming larva and the pole at which fixation 

 takes place. 



Origin of Ova. — The elements which give rise to the germ 

 cells in calcareous sponges have formed a subject for much con- 



