The science of life : fully illustrated in tone and line and including many diagrams

HOW INDIVIDUALS ORIGINATE

From this method, two other main methods of asexual reproduction have been derived. One is multiple fission. In this, the single individual divides into many small ones, either by a series of binary fissions following immediately one upon the other without time for any growth between, or, more frequently in single-celled forms, the central nucleus alone divides repeatedly, and the cell as a whole then splits up simultaneously into as many miniature cells as there are nuclei. The parent individual becomes a mass of smaller cells. This method is frequent with single-celled creatures which growcomparatively speaking—to a large size, such as Foraminifera, or the common large Ameeba, and is found also in the malarial parasite.

The other method consists in making the two products of division unequal. Sometimes, as in various Flatworms and Annelid worms, reproduction is by transverse division, but the hinder individual is at first small and unformed, and only gradually grows and develops. This method has the biological advantage that the original individual retains its individuality, its head and general-alertness, while giving rise to a new specimen from the less specialized tissues of its hinder parts (see Fig. 97).

Sometimes we find this process several times repeated, and then whole chains of incompletely developed individuals may arise behind the original head, trailing after it until growth has developed them far enough to be detached.

Another variation of this method is adopted by the polyp form from which, in many species of jelly-fish, the adult meduse arise. Instead of a string of offspring trailing behind a moving parent, the new individuals appear upon the parent as a fixed

J 7 7 ies tylonychi he most elaborately u f eee aavanlsee here lies See oe it has various large bristles which serve instead of

limbs, and (upper right) a row of triangular vibrating plates to make a foodcurrent. (B-E)

in retaining the fixed attachment ged). during the whole process. The

following suit, leaving the parent stalk

behind to divide and divide again.

When the size of parent and offspring is

E

Fig. 157. How a Complicated Cell Divides.

organized ciliate Protozoa (much

Various stages in its division (diagrammatic). The bristles

igi Paani re black. The original fixed polyp (which de- of the old animal are dotted, those of the two daughter animals are blac

; bs - -oducing region is transversely velops fro t ertiliz o@) current-producing region 1s rsely) ps from the fertilized egg) enlarge, and move to their right p

becomes divided by a series of together, transverse grooves into what looks like a pile of miniature saucers, frilled and lappeted at their edges. The top one of these is the most advanced in development, and is the first to free itself and swim away as a baby medusa, the rest one by one

oe. See hter-cll, while the hind daughter-cell forms a new one.

shaded. The new bristles appear close laces, while the old bristles shrink

The old current-producing region is kept by the front

markedly unequal we speak of budding. “This is rare in single-celled types, but is the commonest form of sexless reproduction in manycelled plants and animals. lis adoption

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