Picture - Cosmic Polymath

DESIGN IN THE REPRODUCTION AND GROWTH OF ANIMALS 373
injured, although it has contributed a quota of its own living matter to the offspring ; that offspring having in
turn the power of reproducing itself by germination. Eeproduction by budding is well seen in colonies of corals.
Like reproduction by division, it is a vital process.
In addition to the foregoing there is reproduction by alternate generations, that is, where two generations are
required to form an individual. The medusa known as Cladonema radiatum affords an example. This animal
is the offspring of the Stauridium or cross-polype of Dujardin. The Stauridium grows upon a stalk hke a polype,
and produces rounded gemmules, which gradually assume the appearance and structure of medusae. In due course
they become detached and float away as medusae. The medusae reproduce themselves sexually, but from the eggs
of the medusae only stauridia can be developed. There is thus an intermediate stage in the reproduction of medusee.
Here again reproduction is unquestionably a vital act. It begins and terminates in living matter which is not
goaded into activity by externalities.
A somewhat similar arrangement obtains in the larvae of certain dipterous insects (Cecidomyia, Miastor), where
maggots produce maggots, and where the first race of maggots is devoured by the second race, which is the only
one to survive, and from which the insects referred to are ultimately produced.
In the case of butterflies there is first the egg, then the grub or caterpillar, then the chrysalis, and finally the
completed insect. Von Siebold has shown that hibernating fertilised female wasps lay eggs in the spring from
which females mainly, with occasional males, are produced. The spring colony of female wasps in turn lays eggs from
which males are developed.
In the case of the tapeworm [Tcenia solium) there are not only alternate generations but two distilnct sexual
organs and elements in each segment of the mature individual. If the fecundated eggs of the tapeworm be eaten
by an animal, say the pig, young tapeworms are developed in the flesh of the pig in the form of cysts, and constitute
the so-called measly pork. This is the first stage in the development of the tapeworm. In the first stage the tapeworm
consists of a bladder-like cyst {scolex) with the head of the future tapeworm plus four curved booklets
inverted or concealed in the body of the cyst. If the measly pork be eaten by man, or other animal, the cysts con-
tained in it in due time develop into tapeworms in the alimentary canal. This they can readily do, and in the
following way. The cysts are introduced into the alimentary canal with the food. Once there the cysts or future
tapeworms evert and free their heads armed with the four curved booklets referred to. By means of the booklets
they fix and anchor themselves in the mucous lining of the alimentary canal, from the contents of which they derive
nourishment by absorption. They grow apace in flat, elongated, quadrilateral segments (proglottides), very small and
slender in the vicinity of the head, but gradually increasing in size as this is receded from. The segments ultimately
measure an inch or more in length and fully a quarter of an inch in breadth ;
each mature segment containing
male and female organs (plus male and female elements), and constituting a separate individual. The adult
segments are crowded with impregnated eggs, and when voided by the anus are in a condition to be carried into any
animal which is unfortunate enough to give them a lodging and act as a host. This is the second and final stage
of development. Two separate animals or hosts are required for the reproduction and continuance of the tapeworm
; but it will be observed that in both the first and second stages of reproduction the reproductive acts are
spontaneous, vital, and in no way due to irritation or extraneous stimulation. The impregnated eggs become
encysted worms in the first host and tapeworms in the second host. The tapeworms take advantage of the two
sets of conditions supplied by the two hosts, and profit by both.
In plants and animals where the male and female organs of generation and elements are situated in different
individuals, something of the nature of coitus or mutual contact is indispensable. The contact in the case of plants
is brought about by winds, insects, and other means. Reproduction by coitus, or its equivalent, can be readily
studied in the fish. If spawning salmon be watched it will be seen that after much serious fighting on the part of
the males for possession of the females, a male and female salmon pair. They proceed to a convenient spot in
the bed of the river, preferably where sand and gravel occur, and scoop out an elongated trough in which the female
fish places herself and deposits her spawn. The male fish in turn occupies the trough and deposits his milt over the
spawn. In some cases the female turns on her side and the male rushes at her and discharges his milt in passing.
The male and female elements commingle, the spawn is fertilised, and the reproductive act completed. It only
remains for the parent fish to cover up the fertilised spawn with sand and gravel to protect it from other fish,
birds, &c. In a short time the young salmon are hatched out, and what is most curious, each young fish is provided
with a bag of pabulum, from which it obtains nourishment until it can develop a mouth, an alimentary canal, and
a vent, when it is in a condition to obtain food from without. The history of reproduction in the fish shows clearly
that the entire reproductive act is vital in its nature, and not only vital but voluntary. In some cases, as in the
viviparous fishes (shark, skate, blenny, &c.), the young are produced alive. Here certainly there is no room for
irritation or extraneous stimulation.