Development of the Marine Sponges. 401 



case, then these sacs are also here foreshadowed, which would 

 account for their great number and full development in the 

 perfected sponge into which the embryo of the species ulti- 

 mately passes. 



Again, should this be right, and we have thus, in the em- 

 bryo of Halichondria simulans, something analogous to the 

 transparent cells in the seed-like body of Spongilla, we shall 

 have to regard the latter as a single ovum, modified in form 

 to meet the circumstances of the case (that is, for preserving 

 the germinative or reproductive substance by a horn-like 

 covering during the dry weather) ; whereas the " swarm- 

 spore " of Spongilla, first described and figured by Lieberkiihn 

 (' Archiv f. Anat. u. Phys.' 1856, pi. xv. fig. 35), being like 

 the soft ciliated embryo of Halichondria simulans, would be 

 for immediate reproduction. 



On referring to the sizes of the seed-like bodies in my 

 description of the five species of freshwater sponges in the 

 island of Bombay (' Annals,' 1849, vol. iv. p. 1), I observe 

 that the diameter of the largest spheroidal form is the same 

 as the long diameter of the embryo of Halichondria simulans, 

 viz. about l-30th of an inch, while that of the other species is 

 much smaller, and that of Spongilla plumosa, whose seed-like 

 body is elliptical, is, in its largest diameter, l-22nd part 

 of an inch. Again, the embryos of the marine sponges Tethya 

 cranium and Tethya zetlandica, which I described and figured 

 (loc. cit. pi. xxii. figs. 4 & 10), were, before leaving the parent, 

 respectively l-24th and l-16th of an inch in diameter. 



If, then, the contents of the embryo of Halichondria simulans 

 can contain and develop a great number of ampullaceous sacs 

 at once, we do not wonder that the seed-like body of Spongilla 

 should contain a great number of transparent spherical germi- 

 niferous cells which also at once pass into the young Spongilla 

 and become ampullaceous sacs — only that they are in the latter 

 developed in advance of the spicules, while in the former the 

 spicules are developed in advance of them. 



Thus, then, my conclusion respecting the " real import " of 

 the seed-like body of Spongilla, at the end of my observa- 

 tions on the subject (' Annals,' 1874, vol. xiv. p. 100) — viz. 

 that it was tantamount to an ovary of which each transparent 

 spherical germiuiferous cell was eqtial to an ovum, and thus 

 immediately passed into an ampullaceous sac as the new sponge- 

 snbstance issued from the seed-like body in the form of the 

 young Spongilla — becomes untenable, as well as the conclusion 

 that " Hiickel's gastrula developed in situ " was only equal to 

 one of these ampullaceous sacs. 



I must therefore fall back upon the term " ovum " for the 



