Development of the Marine Sjionges. 329 



in sea-water (for sea-water must always be used for these 

 purposes to keep the parts alive), we may observe, with £-inch 

 compound power, ova from the l-3000th to the l-1000th inch 

 in diameter (fig. 3, a, V) ; in the smallest of which (a) the yelk 

 and nucleus are already visible, although the nucleolus and 

 germinal vesicle can hardly be distinguished with this power 

 until they have become a little larger (b), while all are re- 

 spectively enveloped in a layer of sarcode, which, being poly- 

 morphic, carries the ovum about with it as an Amoeba does its 

 nucleus (fig. 3,. c). 



At this time the ova appear to be imbedded in the sub- 

 stance of the Halisarca, while the smallest size only exceeds 

 by one quarter the diameter of the spongozoon (fig. 2, a). 

 How they escape from this position, except by their reptant 

 power, I do not know. 



But if we take a horizontal slice off the inner surface of a 

 Grantia compressa, and place it on a slide in a little sea-water 

 with a glass cover over it, we may see the ova, when they are 

 not more than 1-1 700th inch in diameter, attached to the sur- 

 face of the excretory canals, where they not only hang pendent 

 in a pyriform shape, but sometimes leave this position and 

 become reptant (perhaps the human ovum creeps clown the 

 Fallopian tubes in this way ?) ; so that we may fairly infer 

 from this that, in Halisarca lobularis, they follow the same 

 course — that is, pass into and become attached to the excre- 

 tory canals. This stage, as already noticed, I have described 

 and figured in Tethya zetlandica {op. et loc. cit.). After a 

 while, however (that is, in the latter position, viz. the excre- 

 tory canals), they increase to the size of 1-1 80th of an inch in 

 diameter (10-1800ths),when the polymorphic envelope appears 

 to become transformed into a capsular covering, which, at one 

 point, adheres to the parent sponge, and thus placenta-like 

 probably continues to nourish the ovum. 



Second Period. 



That from the first duplicative division of the yelk to its 

 ultimate duplicative subdivision. 



In the second period I have above stated that the ova are 

 found to be much enlarged and congregated upon the rock at 

 the base of the sponge, where they appear in all stages of de- 

 velopment, from the first duplicative division of the yelk 

 (figs. 4 to 10 inclusively) to the perfect embryo (figs. 11 & 12); 

 hence it is only necessary to place a portion of the sponge 

 bearing the ova in this condition under 4-inch compound 



