Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/429

ANIMAL.] REPRODUCTION 411 presented by some of the Amphibians, notably by the Frog Alytes obstetricans, where the male winds the string of ova as laid round his body, sits in concealment until their Fro. 2 (after Sir Wyville Thomson, and Murray, Challenger " Narrative). Sea- urchin (ffemiaster cavernosim, Kerguelun Islands) and Sea- cucumber (Clado- dactyla rosea, Falkland Islands), bearing their young. development is sufficiently advanced, and then takes to the water, or in the grotesque Surinam Toad (Pipa suri- namensis), where the male places the ova one by one in hollows in the loose skin of the back of the female, where they accomplish their development. Reptiles rarely show any care beyond at most burying their ova, but in Birds the evolution of parental care (no doubt associated with the need of high temperature for development) approaches its highest and most general evolution. The case of Mam- malia is also too familiar to need any description; but there is evidently good ground for the idea (of late ably popularized by Miss Buckley) that the marked success of Birds and Mammals in the struggle for existence is to be attributed perhaps not less to their peculiar care of offspring than to any advance in organization. (5) The Spermatozoon. History. In 1677 Leeuwenhoek's attention was drawn by Hamm, one of his pupils, to the active moving con- stituents of the seminal fluid, and he described these as animalcula spermatica or spermatozoa (o-7re'p//,a, seed ; wov, animal). Although known to be of constant occurrence, they were long regarded as parasites, and classified as Hdminthes or as Infusoria (see also article " Zoophytes," Encyclopedia Britannica, 8th ed.), even Von Baer main- taining this view as late as 1835. Soon after this date R. Wagner demonstrated their constant presence in fertile males and their absence in infertile bastards. Von Siebold discovered them in many Vertebrates, while Kolliker recognized them as definite histological elements arising within the testes. The old name, however, has persisted despite various proposals to replace it (e.g., spermatozoids, Von Siebold ; fila spermatica, Kolliker). (.6) Anatomy and Physiology of Spermatozoa. Without attempting completeness it is necessary to note a few of the best marked forms of spermatozoa. The familiar type, that of a small more or less ovoid "head," with delicate thread-like cilium or "tail," is of the commonest occurrence throughout the animal kingdom ; it is seen with specific modifications in 1-4 and 12-16 in fig. 3. Much less differentiated forms, however, occur, especially among the Branchiate Arthropods, of which some exhibit almost amoeboid forms, as in the Daphnoid Moina, or in Crabs (7 in fig. 3); others are rigid, with radiating processes, as in the Lobster (8 in fig. 3). That of Ascaris is a small nucleated cell without tail or process, but bearing a cap of protoplasm ; this, like the lateral process in the spermatozoon of the Cheetopod Cleta (4 in fig. 3), is not, as might at first appear, an additional or accessory portion, but a mere persistent FIG. 3. Forms of Spermatozoa. 1, Sponge; 2, Medusa; 3, BothriocepJtalus; 4, Cleta (CliEetopod); 5, Ascaris; 6, Moina (Daphnid); 7, Crab; 8, Lobster; (9-11), Playiostomuin forms with elongated nucleus; 12, Salamander; 13, Kay; 14, Man ; 15, Cobitis; 16, Mole. embryonic structure, as will be understood from the out- line of the facts of development given below. In a large number the nucleus is more or less drawn out to form either a continuous rod or a series of fine granules, the protoplasm in such cases forming a delicate film with lateral fringes, which may also be produced on one or both ends into a filament. This form is seen in many Planarians (compare 9-11 in fig. 3). In the Newt the head is elon- gated, and the tail bears a vibratile membrane. In Man a middle piece separates the peculiarly shaped head from the tail, and this seems to end in a still finer filament (14 in fig. 3). Miescher and others describe their minute struc- ture with yet greater detail, distinguishing in the head a wall enclosing peculiarly differentiated contents, in the middle piece a similar structure, and even an axial fila- ment in the tail. The movements of spermatozoa have attracted consider- able attention, their action being mechanically comparable to that of cilia, and being affected similarly by reagents. Their great activity and prolonged vitality are note- worthy ; thus not only do they remain in movement for many hours indeed until the onset of putrefaction after their ejaculation from the body, but in many of the higher animals they may remain active in the oviducal passages for weeks. In the Bat the spermatozoa lie in the uterus awaiting the ovum from autumn till spring; while a queen Bee may go on laying the fertilized eggs (from which workers arise) for swarm after swarm for three years after her first and only fertilization, without entirely exhausting the supply of active and mobile spermatozoa in the, receptaculum, and such cases forcibly suggest that the spermatozoon has not only a vast intrinsic store of energy, but must also absorb nutritive matter from the environing tissues or secretions, much in fact as would an Infusorian parasite. (6) The chemical analysis of spermatozoa, though as yet but rude, yields results of interest. Thus, not only is the proportion of solid matter to water extremely high, but the small quantity of ordinary albumens and the high percentage of ethereal extractives are to be noted, the whole composition being more analogous to that of brain and nerve than of any other tissues. In round numbers, nearly 50 per cent, of the Salmon milt is nuclein, more than 25 per cent, is protamin, 10 per cent, is a mixture of ordinary albumens, and less than 5 per cent, is fat ; while 7ij per cent, of lecithin, 2^ per cent, of cholesterin, with traces of other products of metabolism, make up the rest. (7) Development of Spermatozoa (Spermatogcnesis). Probably no subject within the whole range of histology (at least if we except that of the structure of striped nrascular tissue) presents so many difficulties, or has been the subject of more prolonged research and controversy ; and it is thus essential to clearness to recapitulate the history of the inquiry in some detail. The modern