Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/174

Rh 164 EMBRYOLOGY In the present article, accordingly, we shall do no more than trace shortly the steps by which the modern science of Embryology has originated and has assumed the important position which it now occupies among the biological sciences. In its scientific and systematic form embryology may be considered as having only taken birth within the present century, although the germ from which it sprung was already formed nearly half a century earlier. The ancients, it is true, as we see by the writings of Aristotle and Galen, pursued the subject with interest, and the in defatigable Grecian naturalist and philosopher had even made continued series of observations on the progressive stages of development in the incubated egg, and on the reproduction of various animals; but although, after the revival of learning, various anatomists and physiologists from time to time made contributions to the knowledge of the fostal structure in its larger organs, yet from the minuteness of the observations required for embryological research, it was not till the microscope came into use for the investigation of organic structure that any intimate know ledge was attained of the nature of organogenesis. It is not to be wondered at, therefore, that during a long period, in this as in other branches of physical inquiry, vague speculations took the place of direct observation and more solid information. This is apparent in most of the works treating of generation during the 16th and part of the 17th centuries. 1 Harvey was the first to give, in the middle of the latter century, a new life and direction to investigation of this sub ject, by his discovery of the connection between the cicatri- culaof the yolk and the rudiments of the chick, and by his faithful description of the successive stages of development as observed in the incubated egg, as well as of the progress of gestation in some Mammalia. He had also the merit of fixing the attention of physiologists upon general laws of development as deduced from actual observation of the phenomena, by the enunciation of two important proposi tions, viz. (1) that all animals are produced out of ova, and (2) that the organs of the embryo arise by new forma tion, or epigenesis, and not by mere enlargement out of a pre-existing invisible condition (Exercitationes de Gene- ratione Animalium, Amstelodami, 1651, and in English by G. Ent, 1853, London). Harvey s observations, however, were aided only by the use of magnifying glasses (perspe- cillse), probably of no great power, and he saw nothing of the earliest appearances of the embryo in the first thirty-six hours, and believed the blood and the heart to be the parts first formed. The influence of the work of Harvey, and of the success ful application of the microscope to embryological investi gation, was soon afterwards apparent in the admirable xesearches of Malpighi of Bologna, as evinced by his com- .munications to the Royal Society of London in 1672, * De 1 It may be proper to mention, as authors of this period who made special researches on the development of the embryo- (1) Volcher Goiter of Groningen, who, along with AHrovandus of Bologna, made a series of observations on the formation of the chick, day by day, in the incubated egg, which were described in a work published in 1 573, and (2) Hieronymus Fabricius (ab Aquapendente), who, in his work De formato fcetu, first published at Padua in 1600, gave an interesting account, illustrated by many fine engravings, of uterogestation and the foetus of a number of quadrupeds and other animals, and in a pos thumous work entitled De formatione ovi et pulli, edited by J. Prevost, and published at Padua in 1621, described and illustrated by engrav ings the daily changes of the egg in incubation. It is enough, how ever, to say that Fabricius was entirely ignorant of the earlier phenomena of development which occur in the first two or three days, and even of the source of the embryonic rudiments, which he conceived to spring, not from the yolk or true ovum, but from the chalazse or twisted deepest part of the white. The cicatricula he looked upon as merely the restige of the pedicle by which the yolk had previously been attached to the ovary. ovo incubato,&quot; and &quot; De formatione pulli,&quot; and more especially in his delineations of some of the earlier phenomena of development, in which, as in many other parts of minute anatomy, he partially or wholly anticipated discoveries, the full development of which has only been accomplished in the present century. Malpighi traced the origin of the embryo almost to its very commencement in the formation of the cerebro-spinal groove within the cica tricula, which he removed frotn the opaque mass of the yolk ; and he only erred in supposing the embryonal rudi ments to have pre-existed as such in the egg, in conse quence, apparently, of his having employed for observation, in very warm weather, eggs which, though he believed them to be unincubated, had in reality undergone some of the earlier developmental changes. The works of Walter Needham (1667), Regner de Graaf (1673), Swammerdam (1685), Vallisneri (1689) following upon those of Harvey all contain important contributions to the knowledge of our subject, as tending to show the similarity in the mode of production from ova in a variety of animals with that previously best known in birds. The observations more especially of De Graaf, Nicolas Steno, and J. van Home gave much greater precision to the knowledge of the connection between the origin of the ovum of quadrupeds and the vesicles of the ovary now termed Graafian, which De Graaf showed always burst and discharged their contents on the occurrence of pregnancy. These observations bring us to the period of Boerhaave and Albinus in the earlier part of the 18th century, and in the succeeding years to that of Haller, whose vast erudition and varied and accurate original observations threw light upon the entire process of reproduction in animals, and brought its history into a more syste matic and intelligible form. A considerable part of the seventh and the whole of the eighth volumes of Haller s great work, the Elementa Physiologies, published at suc cessive times from 1757 to 1766, are occupied with the general view of the function of generation, while his special contributions to embryology are contained in his Deux Memoires sur la formation du Coeur dans le Poidet, and Deux Memoires sur la formation des Os, both published at Lausanne in 1758, and republished in an extended and altered form, together with his &quot; Observations on the early condition of the Embryo in Quadrupeds,&quot; made along with Kiihlemann, in the Opera Minora (1762-68). Though originally educated as a believer in the doctrine of &quot; pre- formation &quot; by his teacher Boerhaave, Haller was soon led to abandon that view in favour of &quot; epigenesis &quot; or new formation, as may be seen in various parts of his works published before the middle of the century : see especially a long note explanatory of the grounds of his change of opinion in his edition of Boerhaave s Praelectiones Aca- demicce, vol. v. part 2, p. 497 (1744), and his Primes Linece Physiologiae (1747). But some years later, and after having been engaged in observing the phenomena of deve lopment in the incubated egg, he again changed his views, and during the remainder of his life was a keen opponent of the system of epigenesis and a defender and exponent of the theory of &quot; evolution.&quot; as it was then named a theory very different from that now bearing the name, and which implied belief in the pre-existence of the organs of the embryo in the germ, according to the theory of encase ment (emboitement) or inclusion supported i&amp;gt;y Leibnitz and Bonnet. (See the interesting work of Bonnet, Con siderations sur les Corps Organises, Amsterdam, 1762, for an account of his own views and those of Haller.) The reader is also referred to the article EVOLUTION in the present volume, for a further history of the change which has taken place in the use of the term iu more recent times.