Page:The American Cyclopædia (1879) Volume VI.djvu/569

 EMBRYOLOGY 561 blood, others that it came from a kind of female sperm, or seminal fluid secreted by the female organs. In 1651 Dr. William Harvey, in his book on generation, first announced the fact that there is no essential difference in the mode of generation between oviparous and viviparous animals, but that " all animals whatsoever, even the viviparous, and man himself not excepted, are produced from ova." But though the truth of this opinion has since been amply confirmed, and its expression (omne animal ex ovo) has now passed into a physiological aphorism, yet it was not intended by Harvey precisely in the sense which is now given to it. Harvey never saw the unimpregnated eggs of the quadrupeds, nor did he have any idea of the real structure and function of the ovaries in these animals ; and in stating the opinion that the young of the vivipara and of man were produced from eggs, he only meant to say that after sexual in- tercourse and conception, the first thing pro- duced in the uterus was not the embryo, but rather resembled an egg ; and that the embryo was afterward formed from this by the pro- cess of growth. In 1672 Regneer de Graaf showed that the ovaries, in women and in fe- malp quadrupeds, were filled with globular vesicles, visible to the eye, similar in appear- ance to the eggs of birds and fishes. These vesicles he pronounced to be eggs; and the organs in which they were found then took the name of ovaries. A century and a half later (1827) Karl Ernst von Baer discovered by the microscope the real egg of the human female and of the viviparous animals, which is con- tained in the interior of the vesicles of De Graaf. These eggs were shown to exist in the ovaries of virgin females, as well as of those in whom sexual intercourse had taken place ; and it was accordingly demonstrated that in all limals and in man the eggs are formed origi- illy in the ovaries of the female, independently of the male, and that these eggs are afterward fecundated and developed into embryos. An- >ther important discovery remained to complete our knowledge on this part of the subject, viz., that of the spontaneous ripening and discharge f the eggs, in quadrupeds and in man. Ne'grier 840), Pouchet (1842), and Bischoff (1843) de- onstrated that the eggs of the female, origi- ally produced in the ovaries, ripen and are discharged, independently of sexual intercourse, at certain regular periods ; and that the impreg- nation of these eggs by the male sperm is a sub- sequent process, taking place after the eggs have left the ovary and entered the Fallopian tubes. The origin of the embryo accordingly takes place in the same manner in all classes of animals, viz. : from an egg, which is produced in the ovary of the female, discharged thence at certain definite periods, and after ward fecun- dated by contact with the spermatic fluid of the male ; and the only real difference between oviparous and viviparous animals is that in the former species (ovipara) the fecundated egg is discharged from the body of the female and deposited in the nest, or other suitable recep- tacle, in which it is afterward hatched ; while in the latter (vivipara) it is retained in the body of the female, and there nourished during the development of the embyro. The egg, at the time of its discharge from the ovary, con- sists of a globular vitellus or yolk, surrounded by a membrane termed the vitelline membrane Mammalian Egg, highly magnified. a. Vitelline membrane. &. Vitellus. c. Germinative vesicle. d. Germinative spot. In the left-hand figure the egg is shown burst open by the rupture of the vitelline mem- brane and the vitellus partly escaping. and containing a spherical vesicle termed the "germinative vesicle," marked with the "ger- minative spot." In very many instances this becomes surrounded, while passing downward through the Fallopian tubes or ducts, with a layer of transparent albuminous matter; as for example, in the eggs of frogs, tritons, &c. In other cases, in addition to the albumi- nous matter, certain membranous coverings are deposited round the egg, of a fibrous and calcareous texture, as in birds and the scaly reptiles. In all instances, however, it is the vitellus which is the essential part of the egg, and that from which the embryo is direct- ly produced. The first change which occurs after the impregnation of the egg is a spon- taneous division or segmentation of the vitellus. The vitellus divides successively into smaller and smaller portions, in such a way as to pro- duce at last a multitude of minute flattened bodies or cells, which are attached edge to edge, and which form accordingly a continuous membrane, which is called the blastodermic membrane. In eggs which have a large-sized yolk, as those of the birds, lizards, and turtles, the formation of the blastodermic membrane begins at a particular spot on the surface of the vitellus, termed the cicatricula, and thence spreads in every direction, so as to enclose gradually all the rest of the yolk. But in those which are of minute size, as in quadrupeds and the human species, the whole vitellus is con- verted into the blastodermic membrane, which after its formation encloses only a small cavity filled with transparent, watery fluid. The blas- todermic membrane then becomes variously al- tered and developed in different parts so as to form the various organs and tissues of the em- bryo. A line or furrow first shows itself in the thickest and most condensed portion, known as the primitive trace. This indicates the future situation of the spinal column ; and the different parts of the vertebra gradually grow around it, forming a chain of cartilaginous rings, with