The doctrine of descent and Darwinism . tion as merely a preparation for actualdevelopment, at any rate the earliest rudimentary larvalconditions of different types may be compared witheach other. The discoveries of the last ten years with referenceto this subject are so numerous, and such strikinganalogies have been advanced, that we must needs gomuch further than, at that time, was possible for VonBaer. It is not merely a question of those generalanalogies in the segregation of tissues from an indiffer-ent rudimentary mass, but of homologies in the distri-bution, form, and composition of the
The doctrine of descent and Darwinism . tion as merely a preparation for actualdevelopment, at any rate the earliest rudimentary larvalconditions of different types may be compared witheach other. The discoveries of the last ten years with referenceto this subject are so numerous, and such strikinganalogies have been advanced, that we must needs gomuch further than, at that time, was possible for VonBaer. It is not merely a question of those generalanalogies in the segregation of tissues from an indiffer-ent rudimentary mass, but of homologies in the distri-bution, form, and composition of the embryos and larvae,of which the after effects are of profound importance EARLIEST CONDITION OF LARVA. 51 to the later and actual typical impress. With this object,let us consider the larva of a calcareous sponge at thestage which Haeckel has designated as the Gastrulaphase. The diagram gives the section of a larva of thisdescription, which at this period is nothing more than astomach provided with an orifice (fig. 5 ^) ; its wall con-. sists of two strata, or layers of cells. The cells of theexternal stratum are distinguished from those of theinner one by their elongated form, and the possession offilaments serving as organs of locomotion. All subse-quent development and differentiation, certainly notvery important in the sponges, may be traced to modi-fications of these two membranes ; the external mem- 52 THE DOCTRINE OF DESCENT. brane (Ectoderm, or Exoderm) and the internal mem-brane (Entoderm). And this phase of the ciHatedlarva, with its twofold strata, its primitive ventral cavityand mouth, recurs in the Ccelenterata, with slight varia-tions in the Echinoderms, in some of the Annulosa, inthe Sagitta, the Ascidians, and the Lancelet. Fromthe analogy of all these animals, and especially of thelast, we shall be able hereafter to derive importantinductions. But if no weight be attached to the presence ofthese filaments of the external layer, which is, moreover,justified by t
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