The porifera and coelentera . ological problem which would appear, from astudy of the literature alone, to be so confused and difficult as theembryonic development of sponges. The difficidty proves, however, tobe due not so much to the nature of the objects themselves as to themany prejudices and preconceived notions with which they have beenstudied. We may commence the account of this chapter in spongemorphology with the life-history of a very simple and typical form, suchas Clathrina hlanca, in which the adult structure is in all respects similarto that of the Olynthus already described. The
The porifera and coelentera . ological problem which would appear, from astudy of the literature alone, to be so confused and difficult as theembryonic development of sponges. The difficidty proves, however, tobe due not so much to the nature of the objects themselves as to themany prejudices and preconceived notions with which they have beenstudied. We may commence the account of this chapter in spongemorphology with the life-history of a very simple and typical form, suchas Clathrina hlanca, in which the adult structure is in all respects similarto that of the Olynthus already described. The embryology of theremaining types may then be studied from a general point of view, bycomparing, first, the various types of larva, and secondly, their meta-morphosis and organogeny. For fertilisation see above, p. 61. 68 SPONGES (a) Development ofClaihrina hhinca.—The ovum undergoes a regularand total cleavage, resulting in the formation of a hollow, ciliatedblastula of oval form. The segmentation cavity is large, and con-. Fio. 57. Uovflopiiient of Clalhriiia hlaitfa .seen as n living object with iiio<ierate inai:iiilicatioii. 1,larva in optical spction ; 2^, pupal stape of tliH first day of lixation, iiutjuiiorpliosiscomplptd ; 2«, a small portion of the siinie a few hours later, showinR a distinet cpitheliiiiii onthe surface ; S, jiupa at the conmieiicenient of the third day after lixation, .showing the youngspicules and the gastral cavity beginning to form ; 4, young sponge with pores and osculum,of the fifth day. , contracted porocyte ; , flat epithelium ; , exjxinded jwrocyte;osc, osculMm ; sliic, spicule. tains a coagulaV)lc fluid ; its wall is composed of a single layer ofcolumnar, Hagcllated cells, with compressed or onion-shaped one point, the future ])ole of the larva, are a pair of SPONGES 69 very large granular cells with vesicular nuclei, which representundifferentiated blastomeres and are destined to give rise to
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