Page:The American Cyclopædia (1879) Volume VI.djvu/570

 562 EMBRYOLOGY transverse and oblique processes, which envel- ope the primitive trace or furrow, and convert it into a closed canal, large and rounded at the anterior extremity, or head, but narrow and pointed at the posterior extremity, or tail. In this canal the brain and spinal cord are formed and complete the development of their various parts. At the same time the remainder of the blastodermic membrane becomes more con- densed and organized, forming the integument and muscles of the chest and abdomen ; and these portions finally unite with each other in front, forming at the point of junction a longi- tudinal or rounded cicatrix, known as the um- bilicus. The alimentary canal, formed in the interior of the abdominal cavity, is at first en- tirely closed ; but two openings are afterward formed, one at the anterior extremity of the body, the other at the posterior. These open- ings become respectively the mouth and anus. In frogs, tritons, and some kinds pf fish, all these changes take place after the eggs are dis- charged from the body of the female. In birds and turtles the segmentation of the vitellus and the formation of the blastodermic membrane are already far advanced at the time the eggs are laid. In the lizards, most serpents, and some kinds of cartilaginous fishes, the develop- ment of the embryo takes place partly while the egg is still in the generative passages of the female, and partly after its expulsion. In a few species of serpents, and in some fishes, the em- bryo is completely developed within the egg in the body of the female, so that the young are finally brought forth alive; while in all the warm-blooded quadrupeds, as well as in the hu- man species, the fecundated egg is also retained in the uterus until the embryo is sufficiently developed to be born alive. In the frog, the eggs are deposited in the early spring, in some shallow pool, freely exposed to the light and air. Immediately after their expulsion the al- buminous matter with which they are sur- rounded absorbs water and swells up into a tremulous and gelatinous mass, which floats near the surface, with the eggs imbedded in its substance. The formation of the embryo then goes on as above described, and the young an- imal, at first curled up in the interior of the vitelline membrane, soon ruptures it and ef- fects its escape. The body is at this time of an elongated form, terminating behind in a nar- row, compressed tail. The integument is cov- ered with vibrating cilia, which produce a con- stant current of fresh water over the surface of the body. Respiration is performed by gills, situated at the sides of the neck, which are at first exposed, but afterward become covered by a fold of integument. The muscular system is very feeble, and the young animal remains nearly motionless, attached by the mouth to the gelatinous matter around the eggs, upon which he feeds for several days. As he increases in size and becomes stronger, he abandons the spawn and swims about freely in the water, feeding upon the juices and tissues of aquatic vegetables. The cilia with which the body was covered dis- appear. The alimentary canal is at this time very long in proportion to the size of the whole body, being coiled up in the abdomen in a spiral form. During the summer lungs are developed in the interior, and the young tadpole frequent- ly comes to the surface to take in air. But the gills also continue in existence, and are still the most active organs of respiration. To- ward the end of the season anterior and pos- terior extremities or limbs begin to grow ; the posterior sprouting externally from each side, in the neighborhood of the anus ; the anterior remaining concealed under the integument, just below the situation of the gills. The tadpole passes the winter in this transition state. The next spring the lungs increase in size, and the gills become less active as organs of respiration. The anterior extremities are liberated from their confinement by a rupture of the integument which covered them, and both anterior and posterior limbs grow rapidly in size and strength. The tadpole at this time, therefore, has both fore and hind legs and a tail. The tail early in the summer be- comes atrophied, and finally withers and dis- appears altogether; while the limbs, and es- pecially the hind legs, grow to a larger size. At the same time, the lungs attaining their full development and the gills finally disappearing, the tadpole is thus converted into a perfect frog, capable of living and moving upon the land as well as in the water. The tadpole swims by the tail and breathes by gills, while the frog swims by the legs and breathes by lungs. Simultaneously with these changes, the alimentary canal becomes very much shorter in proportion to the rest of the body, and the frog becomes carnivorous in its habits, living principally upon insects, which he is enabled to capture by the great development of his muscular system, and the rapidity and sudden- ness of his movements. The process of de- velopment of the embryo consists, accordingly, in the successive formation and disappearance of different organs which are adapted to dif- ferent modes of life. When these changes take place after the young embryo has left the egg, as in the case of the frog, and so produce marked alterations in the external form of the body, they are termed transformations or met- amorphoses. Thus the egg of the butterfly, when first hatched, produces a caterpillar, or larva ; an animal with a worm-like body, slug- gish crawling movements, and no sexual appa- ratus, but furnished with largely developed di- gestive organs and a voracious appetite. This condition is succeeded by the pupa state, in which the animal changes its skin, losing the legs and bristles which were his locomotory organs, and becomes motionless and nearly insensible to external impressions, and stops feeding altogether. During this period an- other integument grows underneath the old one, with new legs and wings ; and when the skin is again changed, the animal appears as a