396 Mr. H. J. Carter on the 



plastic nature and prehensile property, would be more likely 

 to serve as a temporary rooting development than the upper 

 or ciliated end, which thus appears to be at first entirely 

 developed for locomotion, as the retraction of the cilia proves 

 when it becomes fixed or stationary. 



But with the analogy of the development of the embryo 

 of the siliceous sponges this does not seem to admit of doubt. 



If Hackel followed the development of the embryo of the 

 calcareous sponges (which he terms " gastrula' 1 '') through its 

 transformations into the perfect sponge, he has given no illus- 

 trations of it in his work. Nothing of this kind is to be found 

 in his l Atlas ' between the figure of the embryo (Taf. xxx. 

 figs. 8 & 9 &c.) and the coloured diagrams (Taf. xx. 

 figs. 3 & 4 &c.) which are intended to illustrate his theory ; 

 while his figures of the embryo of the calcareous sponges that 

 he examined at Lesina (op. cit. Atlas, Taf. xxx. figs. 8 & '9 

 &c.) bear upon them a feature which, although it may suit 

 Hackel's theory, is not in accordance with fact. I allude to 

 the direction of the cilia, which are all made to flow " back- 

 wards " or from the obtuse or unciliated end, as if the embryo 

 progressed with this end foremost. (Lieberkuhn has done the 

 same ; but Schmidt not so, /. c.) I need hardly add that this 

 is not the case (PL XX. figs. 13 & 15). Living cilia always 

 lie in the opposite direction to that of the progress of the body 

 of which they are the locomotive organs. 



What position these cilia may occasionally have when 

 the embryo is obtained by forced expulsion after the manner 

 mentioned, I am not prepared to say, beyond the fact that, 

 even in such immature embryos, I have never seen the cilia 

 in the position figured by Hackel. When, therefore, I stated 

 in the ' Annals ' (1874, vol. xiv. p. 98) that " Hackel's illustra- 

 tions could hardly be too highly praised " &c, I had merely 

 studied the embryo of the calcareous sponges by scratching it 

 out on a slide in sea-water from a Orantia compressa which was 

 then pregnant with them, not having seen until lately (that is, 

 since I have followed the development of the ovule in the sili- 

 ceous sponges) the necessity of viewing it as it leaves the 

 parent in the natural way. Hence my opinion of the correct- 

 ness of Hackel's illustrations has undergone much modification; 

 for, beautiful as they must be admitted to be in an artistic 

 point of view, I cannot now help stating respecting them, that 

 " pictures are not always proofs ! ' 



Moreover, by accepting Hackel's views at that time, I took 

 for the endoderm of the embryo in the calcareous sponges 

 that which (now I have had the opportunity of following the 

 development of the embryo in the siliceous sponges) must, I 



