334 Mr. II. J. Carter on the 



this sponge to grow on the under surface of the rocks, I clropt 

 into the bottom of the vessel a small, dark, subangular pebble 

 of quartzite, about § inch in diameter, obtained from the centre 

 of a piece of red-sandstone conglomerate in order that it might 

 be free from impurities, and shortly afterwards observed that 

 one of the embryos had become fixed to the under part of an 

 overhanging portion of this pebble, which in no position ever 

 risked the crushing of the embryo by resting upon the glass. 

 This pebble with attached embryo (Plate XXII. tig. 28, d) was 

 immediately transferred to another finger-glass of the same kind, 

 and the remainder of the embryos left to themselves. As it 

 might be confusing to state any more about the latter here, I 

 defer this for a future opportunity. 



Fourth Peeiod. 



That from the stationary position of the embryo to the deve- 

 lopment of the perfect sponge. 



Having now a fixed embryo, I placed it under an inch 

 compound power, and observed that it had become attached by 

 the posterior end (PL XXII. fig. 28) , which still presented the 

 brown-yellow-coloured mass of cells that distinguishes this 

 point (c), and was further marked by the circlet of long cilia 

 that, undergoing retraction, still played languidly round the 

 base (e), while the whole of the ectodermal cilia had become 

 retracted (a), leaving the embryo with a white, smooth, even 

 surface, and the papillary projection still on the summit (b). 



(Here it should be observed that, whenever the embryo was 

 subjected to examination, the pebble was turned over or to one 

 side, so that both the upper and lateral surfaces of the embryo 

 might be seen and measured respectively, while at the same time 

 this was always done under water, out of which the embryo was 

 never taken until after it had become developed into the perfect 

 sponge. Finding, too, that I was obliged to use another micro- 

 scope for this, in order to get the vessel in between the table 

 and the object-glass, and that this necessitated my using another 

 micrometer, whose divisions were equal to the 1 -830th of an 

 inch, instead of the one I had been using, whose divisions were 

 equal to only l-1800th of an inch, while if the measurements 

 had been reduced to the latter and the objects drawn to this 

 scale, in accordance with those on the previous plates, they 

 would be inconveniently large for this one, I determined to 

 draw them on the scale first mentioned, allowing 1-1 2th to 

 l-830th of an inch, by which it will be seen, and should be 

 remembered, that they are on a scale which makes them a 

 little less than half the size they would be if drawn upon the 

 scale first used, viz. l-12th to l-1800th of an inch.) 



