The reproductive individuals have undergone an extraordinary simplification of the organs concerned with the collection and digestion of food.
In subsequent generations produced by self-fertilization of the hybrids it was found that the positive character was not present in all the individuals, but that a result was obtained showing that in the formation of the reproductive cells (ova and sperms) of the hybrid, half were endowed with the positive character and half with the negative.
The reproductive cells may be regarded as belonging primarily to neither ectoderm nor endoderm, though lodged in the ectoderm in all Hydromedusae.
Insect metamorphosis may be briefly described as phenomena of development characterized by abrupt changes of appearance and of structure, occurring during the period subsequent to embryonic development and antecedent to the reproductive state.
AaK6s, a bag), a botanical term for the membranous sacs containing the reproductive spores in certain lichens and fungi.
Reproductive organs with ducts leading to the vestibule.
After the second larval moult, he passes through a passive stage comparable to the pupa-stadium of an b insect, and during this stage, which occurs inside the root, the reproductive organs are perfected.
Perhaps the earliest of such specializations is connected with the reproductive function.
- In the Hydromedusae the medusa-individual occurs, as already stated, in one of two conditions, either as an independent organism leading a true life c2 a2 in the open seas, or as a subordinate individuality in the hydroid c colony, from which it is never set free; it then becomes a mere reproductive appendage or gono- phore, losing suc FIG.
In this way we may distinguish, first, vegetative reproduction, the result of discontinuous growth of the tissues and cell-layers of the body as a whole, leading to (I) fission, (2) autotomy, or (3) vegetative budding; secondly, germinal reproduction, the result of the reproductive activity of the archaeocytes or germinal tissue.
Balfour put forward the view that the polyp was the more primitive type, and that the medusa is a special modification of the polyp for reproductive purposes, the result of division of labour in a polypcolony, whereby special reproductive persons become detached and acquire organs of locomotion for spreading the species.
- Female reproductive system ofHe/iodri/us.
The fact that in the formation of the reproductive cells of the hybrid generation the material which carries the positive quality is not subdivided so as to give a half-quantity to each reproductive cell, but on the contrary is apparently distributed as an undivided whole to half only of the reproductive cells and not at all to the remainder, is the important inference from Mendel's experiments.
The reproductive organs consist of the parts shown in fig.
The blood of most species behaves differentially towards precipitants, and it is therefore conceivable that when blood is used as food and is elaborated into special compounds for the nutrition of the reproductive organs of a parasite, these specific or larger differences in the blood of animal hosts may prevent the ripening of the gonads of a widely diffused parasite and only one particular kind of blood prove suitable.
Reproductive organs not continuous with ducts.
Both kinds of reproductive organs may occur in a single zooecium, and the reproductive elements pass when ripe into the body-cavity.
In this way the medusa sinks from an independent per sonality to an organ of the polyp-colony, becoming a so-called medusoid gonophore, or bearer of the reproductive organs, and losing gradually all organs necessary for an independent existence, namely those of sense, locomotion and nutrition.
(From Moseley.) irregular coenosarcal canals, the coenosteum may contain, in its superficial portion, chambers or ampullae, in which the reproductive zoids (medusae or gonophores) are budded from the coenosarc.
The cormus is always differentiated into two parts; an upper portion termed the nectosome, in which the appendages are locomotor or hydrostatic in function, that is to say, serve for swimming or floating; and a lower portion termed the siphosome, bearing appendages which are nutritive, reproductive or simply protective in function.
Filicales and Gymnosperms, and known as the Cycadofihices, a group in which, curiously enough, the reproductive organs remained undiscovered for some time after the anatomy of the vegetative organs was sufficiently well known to afford clear evidence of their true affinities.
In a differentiated body the stem (caulome) is an axis capable of bearing leaves and (directly or indirectly) the proper reproductive organs.
The root is an axis which never bears either leaves or the proper reproductive organs (whether sexual or asexual) of the plant.
Flora), the primitive leaf was a reproductive leaf, a sporophyll, from which the foliage-leaf was derived by progressive sterilization.
It is, however, though doubtless near to the base of the Oligochaetous series, most nearly allied in the reproductive system to the Oligochaeta.
Accessory glands are commonly present in connexion both with the male and the female reproductive organs.
- From the mesoderm most of the organs of the body - muscular, circulatory, reproductive - take their origin.
(3) An individual in which the reproductive organs and powers are functionally absent becomes one in which these structures and powers are the only reason for existence, for the great majority of insects die after a brief period of reproduction.
The reproductive on the lateral vessel of system is of the simplest, strongly con- Drepanophorus spectatrasting with the complicated arrange- bilis.
There are no specialized sense-organs or vascular or respiratory systems. There is a wide body-cavity, but as this has no connexion with the renal or reproductive organs it cannot be regarded as a coelom, but probably is a blood-space or haemocoel.
Thus the whole of the Pulmonata (which breathe air, are destitute of gill-plumes and operculum and have a complicated hermaphrodite reproductive system) are either snails or slugs.
Eleven abdominal segments can be recognized, at least in the early stages; as the adult condition is reached, the hinder segments become reduced or modified in connexion with the external reproductive organs, and show, in some male Hemiptera, a marked asymmetry.
And though Spencer's general position - that it is absurd to suppose that organisms after being modified by their life should give birth to offspring showing no traces of such modifications - seems the more philosophic, yet it does not dispose of the facts which go to show that most of the evidence for the direct transmission of adaptations is illusory, and that beings are organised to minimize the effects of life on the reproductive tissues, so that the transmission of the effects of use and disuse, if it occurs, must be both difficult and rare - far more so than is convenient for Spencer's psychology.
In other cases the reproductive cells perhaps pass out by the atrophy of the polypide, whereby the body-cavity may become continuous with the exterior.
Speaking generally each species of parasite has a particular host, upon the blood of which it nourishes itself and matures its reproductive organs.
(Xioo.) reproductive system; C, Cirrus; H, hooks on the ventral sucker; I, small piece of the intestine to show its connexion with the reproductive organs by the narrow duct that passes from it to the union of the vaginae; M, mouth; 0, ovary; S, oral sucker; SC, sucker; SH, shell-gland; T, Testis; U, uterus; V, vaginal pore; Y, yolkgland.
- A, reproductive system of Amphilina foliacea: a, glandular pit; b, opening of uterus; b', uterus (black); c, yolk-gland and its duct; d, ovary; e, e', opening and duct of vagina; f, spermotheca; g, male genital opening (gonopore); h, penis; i, vas def erens; j, testes; k, shell-gland.
The reproductive organs are usually repeated in each proglottis, and in some families two complete sets of such organs occur in each segment; in a few cases, parts only of the system are duplicated.
The chief difference between the reproductive organs of the two classes is the presence in Cestodes of a separate vagina and uterus, each of which opens in some families to the exterior by an independent pore.
In fact, the whole history of the Platyelmia is marked by a great specialization of the reproductive evolutionary history, accompanied by a simple somatic line of evolution.
1 Thus they are pro vided with a nucleus which is the centre of cell activity; Pathological both of the reproductive and chemical (metabolic) pro- GelIs.
The failure of the material carrying a positive character to divide so as to distribute itself among all the reproductive cells of a hybrid individual, and the limitation of its distribution to half only of those cells, must prevent the swamping " of a newly appearing character in the course of the inter-breeding of those individuals possessed of the character with those which do not possess it.