OVUM. 



45 



serrations and doctrines of Schwann, as 

 to the cellular origin and constitution of 

 the textures of plants and animals, published 

 in 1838, have exerted on the progress of ovo- 

 logy and embryology. In accordance with 

 his general views, Schwann regarded the 

 ovarian ovum as constituting an organised 

 cell, of which the vitelline membrane is the 

 outer cell-wall, the yolk substance the con- 

 tents, the germinal vesicle the nucleus, and the 

 macula or maculae the nucleolus or nucleoli.* 

 While the observations of Schwann do not 

 present us with any very important new facts 

 as to the structure of the ovum in particular, 

 and although it may be necessary even to 

 modify, in some degree, the view we should 

 now, with our increased knowledge, be dis- 

 posed to take of its relation to other organised 

 cells of the economy, still it cannot be doubted 

 that the great generalisation matured by this 

 author has exerted, and still continues to 

 exert, upon the progress of discovery and the 

 scientific aspect of our subject, a no less re- 

 markable influence than that which has been 

 apparent since its first promulgation, on al- 

 most the whole range of physiological ana- 

 tomy ; furnishing the key to the structure of 

 the 'ovum itself, laying the foundation of the 

 histological laws of development, bringing a 

 vast amount of observed facts under a con- 

 sistent doctrine, and stimulating and guiding 

 the embryologist in new paths of research.f 



* Schwann, after hesitating somewhat whether 

 to regard the entire ovum or the germinal vesicle as 

 a true cell, gave the preference to the above view, 

 chiefly from a consideration of the manner in which 

 the ovum was believed to be formed, and its corre- 

 spondence, as he conceived, with the invariable and 

 necessary mode of formation of cells in general. 

 These latter views have themselves undergone some 

 modification since the publication of Schwann's 

 Researches, and we shall probably find (as. will after- 

 wards be more fully stated) that a study of the 

 structure and mode of production of the ova in a 

 variety of animals, may render it necessary to mo- 

 dify also the comparison of the parts of the ovum to 

 the organic cell, adopting rather the view of R. 

 Wagner and Henle, according to which the germinal 

 vesicle is regarded as the true cell, and the other 

 parts of the ovum as of the nature of superadded 

 structures. 



t Mikroskop. Untersuch. uber die Ubereinstim- 

 mung in der Struktur uud dem Wachsthum der 

 Thiere und Pflanzen, von Dr. Thomas Schwann. 

 Berlin, 1839. The interesting Researches of M. J. 

 Schleiden, on the cellular structure of Plants, en- 

 titled Beitrage zur Phylogenesis, published in 

 Muller's Archiv. for 1838, immediately preceded 

 those of Schwann ; and they may be considered as 

 forming in some measure, along with his, the basis 

 of modern histology: the researches of the latter 

 author were independently made, and an extract 

 of them published in the beginning of 1838, in 

 Froriep's Notizen ; and the two first parts of his fuller 

 treatise were sent to the French Academy, in August 

 and December of 1838. An interesting analysis of 

 both these works in the British and Foreign Medical 

 Review, vol. ix. for 1840, made their contents 

 generally known in this country ; and an excellent 

 translation of them both by Mr. H. Smith has since 

 been published (1847) under the auspices of the Sy- 

 denham Society. It may be observed, however, 

 that the great and important generalisations which 

 Schleiden and Schwann have, with so much merit, 



The most important general facts that have 

 been ascertained in regard to the ova of ani- 

 mals may now be stated shortly as follows : 



1. The ova of animals begin to be formed 

 in the ovary of the female parent at an early 

 period of life, and are produced more com- 

 monly in vesicular, more rarely in tubular, 

 cavities or 'ovisacs of these organs. 



2. In those instances, in which it has been 

 possible to detect the earliest stages of forma- 

 tion, the ovum appears very generally to take 

 its origin in the form of a minute spherule or 

 vesicle. This soon enlarges, and is converted 

 into a nucleated cell, which forms the germinal 

 vesicle, and this remains as a constituent part 

 of the ovulum, till the latter is about to leave 

 the ovary, or till the first formative changes 

 commence. 



3. The vitellus, or yolk substance, begins 

 to be formed a little later. It is gradually 

 accumulated, either in the vicinity of or round 

 the germinal vesicle, in some rarer instances 

 being produced in a separate organ ; it con- 

 sists of an albuminous fluid in which are 

 suspended a large quantity of granules and oil 

 spherules, or of larger corpuscles, having the 

 form of non-nucleated simple cells. The 

 structure of the yolk substance is subject to 

 considerable variety. 



4. The yolk substance and germinal vesicle 

 are together enclosed, in the mature state of 

 the. ovarian ovum, by an almost structureless 

 vesicular vitelline membrane, which in most 

 animals is formed later than the other parts, 

 in some at an early period, in others com- 

 paratively late, and which is not, therefore, 

 to be invariably regarded as a true cell-wall, 

 or as an original part of the ovum. 



5. The germinal vesicle presents several of 

 the characteristics of a true organised cell : 

 the macula, or germinal spot, has in many 

 animals the form and relation of a nucleus ; 

 but in some, the subdivided structure of this 

 macula, which succeeds in the progress of 

 formation of the ovum, causes it to lose its 

 simple nuclear character. 



6. When the ovulum, or ovarian ovum, 

 attains maturity, it generally leaves the place 

 of its formation in the ovary, and about the 

 same time in some animals before, in others 

 after it the germinal vesicle disappears, bursts, 

 or dissolves. In the greater number of ani- 



established, may in some measure be regarded as the 

 fulfilment or amplification of, and true deduction from, 

 a variety of detached observations, which had been 

 accumulating, in regard to both kingdoms of 

 nature, for some years, more especially after the im- 

 provement of the achromatic microscope had given 

 increased facility and precision to the minute ex- 

 amination of the tissues ; such as the discovery of 

 the nucleus of the vegetable cell by Robert Brown, 

 in 1831 ; the discovery of the cellular structure of 

 the chorda dorsalis by J. Muller ; the observations 

 of Henle on epithelium, of Valentin on several similar 

 subjects, and of Turpin and Mirbel on plants, together 

 with the observations previously referred to on the 

 ovum of animals itself. But these detached ob- 

 servations had not led to any important general 

 views before the publication of the Researches of 

 Schleiden and Schwann. 



