Page:The American Cyclopædia (1879) Volume VI.djvu/574

 566 EMBRYOLOGY While, however, in the vertebrate animals, the embryo always lies with its belly upon the sur- face of the yolk, in some of the invertebrates, as the articulata (insects, spiders, crustaceans), the back of the embryo is in contact with the yolk, and the closing up or union of the two sides of the body takes place along the dorsal line, instead of the abdominal. In many mol- lusks, as for example in snails^ the embryo, soon after the commencement of its formation, begins to rotate slowly in the interior of the vitelline sac ; and this rotation continues more or less rapid until the hatching of the egg. In the invertebrate classes the metamorphoses or transformations of the young animal are more frequent and more striking than in vertebrata. In many of them the young animal when first hatched from the egg is entirely unlike its parent in structure, external appearance, and habits of life. In the class of insects many of these transformations are well known, and have always attracted the attention of the curi- ous. Frequently the young animal, in passing through several successive transformations in which he is adapted to different modes of life, necessarily changes his habitation ; and being found accordingly in totally different localities, and presenting at successive intervals corre- sponding differences of organization, the same embryo at different ages is often mistaken by the ignorant for an entirely distinct species of animal. These changes of habitation, occurring in the course of embryonic development, are termed migrations. They are often very mark- ed in parasitic animals. Thus the tsenia, or tapeworm, inhabiting the small intestines of certain animals, such as the dog, cat, &c., pro- duces an egg containing a small globular em- bryo, armed with certain hard spikes, or curved prominences, capable of being moved by mus- cular fibres inserted into their base. The por- tion of the tapeworm in which these eggs are contained, known as the proglottis, is discharged from the intestine of the first animal, and the eggs, becoming mixed with vegetable matter, are devoured by animals belonging to other species, as for example the pig. Either in the process of mastication, or by the action of the digestive fluids of the stomach, the external envelope of the egg is destroyed, and the em- bryo set free. By means of its movable pro- jecting spines, the embryo then makes its way through the walls of the stomach or intes- tine into the neighboring organs, and often reaches distant parts of the body. Here, be- coming arrested, it is temporarily fixed in place by the consolidation of the tissues round it, and becomes enlarged by the imbibition of fluid, assuming a vesicular form. A portion of this vesicle Becomes inverted, and at the bottom of the inverted part a head is produced, upon which there are formed four muscular disks, or suckers, and a circle of calcareous spines or hooks, different from those present at an earlier period, which are thrown off and lost. In this state the animal receives the name of scolex or cysticercus. It remains in that condition till the death of the animal whose tissues it in- habits, when, being devoured with the flesh by an animal belonging to the first species, it passes into the intestine of the latter, and there be- comes developed into the complete tapeworm, or strobila, similar to that from which its em- bryo was first produced. The same animal is accordingly a parasite in different organs, and even in different species, at different periods of its development. Some of the invertebrata are parasitic at one stage of their existence, and lead an independent life at another. Such are the small Crustacea which infest the bodies and gills of certain fishes. In the family of cestridce, or bot flies, the eggs are deposited by the female insect, and attached to the hairs of horses, cattle, &c. ; from which situation, after the embryo has become partly developed, they are detached in some instances (as in gastro- philus equi) by licking, and swallowed into the stomach. Here the larva is set free, and attaches itself to the mucous membrane of the stomach, nourishing itself upon the fluids ob- tained from this source, and gradually increas- ing in size. After a certain period the larva lets go its hold, passes through the intestine, is discharged with the fffices, and, assuming the pupa state, is finally transformed into the per- fect insect. The process of embryonic devel- opment is accordingly a succession of changes, in which the structure and organization of the young animal are adapted to different modes of existence, and in which different organs and apparatuses, successively appearing and disap- pearing, replace each other in the progress of growth, and give rise to the appearance of transformations, which affect the body as a whole. See Harvey, Exercitationes Anato- micm de Generations Animalium (London, 1651 ; Sydenham edition, London, 1847); Spal- lanzani, Saggio di osservazioni microscopiche, &c. (Modena, 1767), Prodromo sopra le pro- duzioni animali (Modena, 1768), and other works ; Baer, De Om Mammalium et Hominis Q-enesi Epistola (Leipsic, 1827), and TJeber En- twiclcelungsgeschicJite der Thiere (1828) ; Valen- tin, Handbuch der EntwicTcelungsgeschichte dea Menschen (Berlin, 1835) ; Coste, EechercTies sur la generation des mammiferes (Paris, 1834), Embryogenie comparee (1837), and Histoire generale et particuliere du developpement des corps organises (1847, '49, '53) ; Pouchet, The- orie positive de la fecondation des mammiferes (1842), and Theorie positive de Vovulationspon- tanee et de la fecondation des mammiferes et de Tespece Jiumaine (1847) ; Bischoff, Bei- fung und Loslosung der Eier der Sdvgethiere und der Menschen (Giessen, 1844), and his treatises on the embryology of the hedgehog (1853), ape (1866), &c. ; Rathke, Ueber die Entwiclcelung der Schildlcrdten (Brunswick, .1848), Flusslcrets (1829), Columber Natrix (1839), Krolcodille (1866), &c. ; Agassiz, "Lec- tures on Comparative Embryology " (Boston, 1849) ; H. Baudrimont and Martin Saint- Ange,