Multiplication by spontaneous fission seems from some recent researches to be much less frequent than has been supposed, and more evidence is required as to the fact of the habitual propagation of _any_ planariae in this fashion.[169] But even if this were as asserted, {169} nevertheless it fails to explain the peculiar condition presented by _Syllis_ and some other annelids, where a new head is formed at intervals in certain segments of the body.

Here there is evidently an innate tendency to the development at intervals of a complex whole.

It is not the budding out or spontaneous fission of certain segments, but the transformation in a definite and very peculiar manner of parts which already exist into other and more complex parts.

Again, the processes of development presented by some of these creatures do not by any means point to an origin through{170} the linear coalescence of primitively distinct animals by means of imperfect segmentation.

Thus in certain Diptera (two winged flies) the legs, wings, eyes, &c., are derived from masses of formative tissue (termed imaginal disks), which by their mutual approximation together build up parts of the head and body,[170] recalling to mind the development of Echinoderms.
[Illustration: AN ANNELID DIVIDING SPONTANEOUSLY.
(A new head having been formed towards the hinder end of the body of the parent.)] Again, Nicholas Wagner found in certain other Diptera, the Hessian flies, that the larva gives rise to secondary larvae within it, which develop and burst the body of the primary larva.