146 ANATOMY AND PHYSIOLOGY 



detect no line of distinction between them, and they con- 

 tinue to grow as one ovum." 



I have never had the good fortune to see the living 

 gemmule with its cilia in action, as described by Dr. Grant; 

 but I have frequently found Halichondraceous sponges with 

 an abundance of these gemmules attached to their tissues ; 

 and I have in my possession a beautiful little specimen, 

 dredged off Shetland, for which I am indebted to my kind 

 friend Mr. Bailee, which is very illustrative of Dr. Grant's 

 description of the mode of the development of the young- 

 sponge after the ovum or gemmule has attached itself. On 

 a fragment of a bivalve shell there are more than twenty or 

 thirty of Dr. Grant's ova or gemmules, which are all in the 

 same early stage of development, each forming a small 

 group of extremely slender spicula. The groups are sepa- 

 rate from each other, but very closely adjoining. The 

 diameter of one of the largest does not exceed ^th of an 

 inch, and their distance from each other is about half or 

 once the diameter of one of them. In their present state, 

 as represented by six of them in Fig. 339, Plate XXIV, it 

 is evident that they are separate developments ; and it is 

 equally evident that a slightly further amount of extension 

 would have caused them to merge in one comparatively 

 large flat surface of sponge. We see by this instance that 

 a sponge is not always developed from a single ovum or 

 gemmule, but, on the contrary, that many ova or gemmules 

 are often concerned in the production of one large indivi- 

 dual ; and this fact may probably account for the compara- 

 tively very few small sponges that are to be found, a few 

 days probably serving by this mode of simultaneous deve- 

 lopment to form the basal membrane of the sponge, of con- 

 siderable magnitude, as compared with the individual ovum 

 or gemmule, or with a sponge developed from a single 

 ovum only. This mode of reproduction appears to have 

 a very wide range. It is common to several distinct genera 

 of Halichondraceous sponges ; and I have observed it also 

 in a siliceo-fibrous sponge, Ipldieon panicea of the Museum 

 of the Jardin des Plantes, Paris. Pig. 340, Plate XXV, 

 represents a small piece from the interior of the skeleton of 



