EMBRYONIC DEVELOPMENT 359 



resembling extremely closely, sponges growing in the West Indies, we can only infer 

 either that their superficial resemblance is due to convergence, which is improbable, or 

 that the possible range of migration, of some species at least, is far greater than has been 

 hitherto believed possible; or, and this is the most probable, that these sponges have 

 also become established in favourable areas, perhaps extremely localized, in the Indian 

 Ocean and the South Atlantic, which have not so far been recorded in the scientific 

 literature. This is, to some extent, borne out by the record (St. i, p. 340) of Spongia 

 officinalis, Linnaeus, from Ascension Island. 



EMBRYONIC DEVELOPMENT AND BREEDING SEASONS 



It is probable that too little attention has been paid in the past to the value of describ- 

 ing embryos found in preserved material. From a purely embryological point of view 

 such data as may be obtained from them is insufficient, but to the systematist the know- 

 ledge gained may be of immense value. Sections cut for purposes of identification may 

 not ordinarily reveal the presence of embryos, which are often situated in the deeper 

 parts of the sponge. For example, hand sections made from a specimen of Tedania 

 temiicapitata, Ridley, in the present collection, revealed no trace of embryos, but these 

 were found in large numbers when the specimen was cut in two. Wherever practicable 

 examination should therefore be made for embryos and their state of development and 

 other cognate information recorded. 



EMBRYONIC DEVELOPMENT 



Since most information has been obtained regarding the development of the 

 Antarctic species of Tedania, as compared with species of other genera, it will be 

 convenient to deal with these first. Numerous embryos were found in some twenty or 

 thirty of the specimens of Tedania examined and from these a fairly complete record of 

 development in this group of species may be obtained. In most cases, the embryos 

 present in a given specimen represent only a limited part of the developmental sequence ; 

 but in a specimen of T. charcoti, Topsent, the whole sequence can be followed in a few 

 hand sections, from the unsegmented ovum up to the time at which the embryo escapes 

 from its capsule. Embryonic development appears to follow the same course, broadly 

 speaking, in all five of the Antarctic species, but differs considerably in such details as 

 the size, shape and behaviour of spicules and in small details of histology. 



The embryos of a single species, as represented by samples from several individuals, 

 vary slightly in size, in the size and shape of the spicules, in the time at which the 

 spicules appear and in the subsequent behaviour of these spicules ; but such differences 

 are small and are for the present purpose negligible. The specimens examined were all in 

 the form of thin hand sections, cleared in clove oil, unstained or stained in borax 

 carmine. In these preparations, the embryos appeared as solid bodies, but in a few 

 instances specimens were found to have been satisfactorily sectioned by the razor. It 



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