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

BOOK 4

confers the advantage of leaving the original individual fully formed and active all the time, dispensing with any need for reby putting

organization,

material of growth direct into rapidly developing buds, which can be thrown off when they are fit to

look after themselves. Budding is widespread among _ sea-anemones,

jelly-fish, sea-squirts and many other invertebrates.

Even when sexless reproduction has been abandoned by the adult, as in all the highest animals, it may persist unsuspected in early stages of life. Generally, every fertilized egg means a new individual, and if two or more individuals appear at a birth, then two or more eggs were fertilized.

The triplets, quartets, quintets and so forth of cats or rabbits arise from groups of separate eggs, three, four, five and so on in number. But there are exceptions. In the Texas armadillo, whose bony carapaces are so often made into baskets to tempt the tourist, four young

are always born at a birth, and _ they are peculiar in sharing a common set of These quadruplets are produced by the early division into four parts of a single fertilized egg.

embryonic membranes.

270

THE SCIENCE OF LIFE

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Fig. 158. How a gemmule is formed in the freshwater sponge, Spongilla (highly magnified).

Top left: Cells packed with nutriment are gathering at one spot ; other cells are beginning to secrete a horny case round them. Top right : The first horny case is finished, and microscopic spicules have been transported to make a further protective layer round it. Bottom: The hornsecreting cells have retreated and made a second horny case round the layer of spicules. The gemmule is now complete.

Ss.

case.

Similarly

all the cases of human (twins which are so closely alike as to cause confusion) are due to the splitting of a single embryo at a very early stage.

@ HAP TER 2

identical twins

This splitting

is, in a perfectly legitimate sense, sexless reproduction. In some parasitic insects this budding of the ovum occurs on a grander scale. Only one egg is laid in the destined victim of the parasite. This divides not into two or four, but into hundreds of cell-masses, from each one of which a whole insect eventually grows.

A very peculiar form of sexless reproduction is found in many sponges. Sponges possess large numbers of wandering cells, not unlike our own white blood corpuscles. A number of these collect together in one spot, much as our white corpuscles

collect where there is inflammation. However, their function is not defence against intruders,

but reproduction, and they have previously accumulated food-stores within themselves. They pack themselves close, and are

then sealed up by other cells in a protective In a single sponge there may be a multitude of these agglomerations. When the parent sponge dies down under stress of bad weather or other discouraging conditions,