The principles of biology . ex-pect that the united individuals wiU habitually indicate theiroriginal independence by severally bearing germ-producingor sperm-producing organs. The reasons for believing one of these creatures to be anaggregate of the third order, are greatly strengthened whenwe turn from the adult structure to the mode of develop-ment. Among the Dorsibranchiata and TuMcolce, the em-bryo leaves the egg in the shape of a ciliated gemmule, notmuch more difierentiated than that of a polype. As shownin Fig. 162, it is a nearly globular mass; and its interior THE MOEPHOLOGICAL COMPO
The principles of biology . ex-pect that the united individuals wiU habitually indicate theiroriginal independence by severally bearing germ-producingor sperm-producing organs. The reasons for believing one of these creatures to be anaggregate of the third order, are greatly strengthened whenwe turn from the adult structure to the mode of develop-ment. Among the Dorsibranchiata and TuMcolce, the em-bryo leaves the egg in the shape of a ciliated gemmule, notmuch more difierentiated than that of a polype. As shownin Fig. 162, it is a nearly globular mass; and its interior THE MOEPHOLOGICAL COMPOSITION OF ANIMALS. 93 consists of untransformed cells. The first appreciable changeis an elongation and a simultaneous commencement of seo--mentation. The segments multiply by a modified gemma-tion, which takes place from the hinder end of the penultimatesegment. And considerable progress in marking out thesedivisions is made before the internal organization 163,164,165, represent some of these early stages. In. /6S AnneHds of other orders, the embryo assumes the segmentedform while still in the egg. But it does this in just thesame manner as before. Indeed, the essential identity of thetwo modes of development is shown by the fact that the seg-mentation within the egg is only partially carried out: iaall these types the segments continue to iacrease in nimiberfor some time after birth. Now this process is as like that by which compound animals in general are formed,as the difierent conditions of the case permit. When newindividuals are budded-out laterally, their unfoldiug is nothindered—there is nothing to disguise either the process orthe product. But gemmae produced one from another in thesame straight line, and remaining connected, restrict oneanothers developments ; and that the resulting segments areso many gemmiparously-produced individuals, is necessarilyless obvious. § 206. Evidence remains which adds very greatly to theweight of that already assigned.
Size: 2292px × 1090px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1860, booksubjectbiology, bookyear1864
