Development of the Marine Sponges. 395 



the embryo of Grantia compressa (although this has not 

 actually been witnessed) does attach itself, to the body (viz. the 

 branch of Ptilota) on which it becomes developed, by the 

 bunch of cells at the base of the cone, and therefore that 

 these are especially provided for rooting the embryo. 



Of the whole of the development of the embryo of Grantia 

 compressa, this, as before stated, on account of the minuteness 

 of the former at this period, has not been seen ; but that the 

 embryo does become fixed in this way, and that the pointed 

 end becomes the mouth [oscidum) or aperture of the cavity of 

 the body in the young* Grantia compressa, and not the obtuse un- 

 ciliated end, maybe reasonably concluded from the comparison 

 between the embryos of Grantia compressa and that of Hali- 

 chondria simidans just instituted. 



Every thing too, after this, points to the same kind of de- 

 velopment in the fourth period as that presented by the 

 embryo of Halichondria simulans in passing into the form 

 of the parent sponge ; so that to make an exception of the 

 mode of attachment because we have not actually seen it 

 in Grantia compressa seems to me, under such circumstances, 

 most unreasonable. 



Lieberkiihn, who, as before stated, described and figured this 

 embryo in 1859 (' Archiv f. Anat. u. Phys.' p. 379, pi. ix. 

 fig. 7), and Schmidt, who did the same in 1866 (' Spong. 

 adriat. Meeres,' 2nd suppl. p. 5, pi. i. fig. 6), have con- 

 sidered the truncated, unciliated end the hinder portion ; yet 

 Hackel (in 1870), in opposition to these distinguished spongo- 

 logists, has turned it upside down ( l Die Kalkschwamme,' vol. i. 

 pp. 336-8, Atlas, pi. xiii. figs. 5 & 6, and pi. xx. figs. 3 & 4 &c). 



Now, supposing that Hackel had not studied the develop- 

 ment of the ovum into the perfect sponge in the siliceous 

 species, and therefore had not the analogy to go by that we 

 have, but had really seen the embryo of the calcareous sponges 

 after it had left the parent in a natural way (that is, not by 

 forced expulsion under the tearing to pieces of the body of the 

 parent on a slide, but, by cutting off a branch of the Ptilota 

 on which the sponge might be growing, and treating it in the 

 way above mentioned) , he ought to have observed that its pro- 

 gression was with the pointed or ciliated end (PI. XX. fig. 13, a) 

 foremost, that the cilia of the ectoderm (fig. 13, b) were thus 

 more or less inclined towards the posterior or truncated un • 

 ciliated end (fig. 13, c/), and that it attached itself to the 

 glass vessel and foreign bodies by this end ; under which cir- 

 cumstances it seems to me that he might have at once concluded 

 that, on becoming fixed, the bunch of large cells at the pos- 

 terior end (fig. 13, d), being without cilia and endowed with a 



