[136] 



OVUM. 



rent isolation of the origin of cells in blastema 

 or intercellular substance, it might still be 

 held that the unseen germs of new cells con- 

 tained in that blastema may have derived 

 their origin from other cells or organised parts 

 proceeding from cells. And thus, in regard 

 to the first origin of the ova of animals, it is 

 fair to conjecture that the germs from which 

 they spring have taken their descent from 

 parent cells or structures derived from cells 

 through the organs appropriate to their form- 

 ation. But here observation fails to assist us 

 further, and we are lost in the region of 

 speculation. 



If, however, with the reservations now 

 stated, it should be thought desirable to 

 compare the ovum to the organic cellular 

 structures, the germinal vesicle may be re- 

 garded as the simple cell of the ovum, the 

 whole ovum as a complex cell ; the first of 

 these being formed probably by expansion 

 from a minute point or molecule, the second 

 by superposition or external deposit round 

 the internal cell ; but both at the same time 

 presenting features which are peculiar to 

 themselves, and different from those which 

 characterise other cells of the animal eco- 

 nomy. The different and separate formation 

 of the germinal vesicle and yolk, which is 

 perceptible to some extent in the ova of most 

 animals, is placed in its most striking point of 

 view by those instances in which, as in Tre- 

 matode and Cestoid Entozoa, there are dis- 

 tinct germigenous and vitelligenous organs, 

 and those in which, as in Nematoidea and 

 Insecta, the ovary is tubular, and the forma- 

 tion of the several parts of the ovum goes on 

 progressively in different parts of the tube. 



4. Phenomena attendant on the maturation 

 of the ovum, and its discharge from the 

 ovary. 



The ovum naturally undergoes in the ovary 

 a progressive development till it arrives at the 

 state of maturity, when it is usually separated 

 from the ovary by a process of dehiscence, is 

 conducted through the female passages either 

 to be excluded or laid, as in oviparous ani- 

 mals, or to be retained in a uterus or other 

 part of the female organs in viviparous ani- 

 mals during uterogestation. The maturation 

 of the ova and their separation from the 

 ovary is in many animals periodical and inde- 

 pendent of fecundation. This natural peri- 

 odical separation of the ova has been termed 

 Ovulation by some authors.* 



The change which the germinal vesicle 

 undergoes at the period of the maturation of 



* The observations of Bischoff had long ago 

 shown that in the periodical dehiscence of ova 

 which accompanies the heat of female quadrupeds, 

 the ova may be detected, though unfecundated, in the 

 course of their descent through the Fallopian tubes 

 and uterus (Periodische Losreifung, &c., 1842), and 

 some observations appear also to have shown that 

 the same is the case in the human female at the 

 periodical return of menstruation. (See a paper by 

 H. Letheby, M. B. in the Philos. Trans, for 1851, 

 p. 57., where two cases are described in which ovules 

 or their remains were detected in the Fallopian 

 tubes of unimpregnated women who had died at or 

 about the menstrual period.) 



the ovum has naturally attracted much at- 

 tention, from the hope that through the ob- 

 servation of this phenomenon some explanation 

 might be obtained of the first origin of the 

 germ round which, after fecundation has taken 

 place, the segmenting and organising stratum 

 is collected, from which the blastoderm is 

 produced ; but it must be allowed that as yet 

 little success has attended our efforts to de- 

 tect the connection, if it exists, between these 

 two processes. In almost all animals the 

 germinal vesicle is lost to view at the time of 

 the maturation of the ovum, and generally 

 before or about the time when the ovum 

 leaves the ovary. In large-yolked ova the 

 macuke of the germinal vesicle become very 

 numerous by their multiplication and sub-divi- 

 sion at an early period ; while in the small- 

 yolked ova, as has been observed in a few 

 animals at least, the increase in the number 

 of the maculae does not take place till imme- 

 diately before the diffluence or disappearance 

 of the vesicle. The more minute phenomena 

 of this diffluence are as yet very imperfectly 

 known. In some animals, as Mammalia and 

 Birds, it has been observed that shortly 

 before the diffluence of the vesicle, its delicate 

 wall undergoes a softening or approaching 

 solution, so as to make it impossible to 

 separate the vesicle entire. After this, when 

 the diffluence is complete, the contents dis- 

 appear from the situation they have previously 

 occupied, but what becomes of them has not 

 yet been determined. In some instances, as 

 Birds and Batrachia, it has been observed 

 that, after the diffluence of the germinal ve- 

 sicle, the germinal part of the yolk, which 

 previously consisted exclusively of small 

 opaque granules, is now mingled throughout 

 with clear hyaline spherules, somewhat similar 

 to the dispersed maculae of the germinal 

 vesicle ; but it is only matter of conjecture 

 that these clear spherules have been derived 

 from the germinal vesicle or its maculae. 



In a few instances, as in Ascaris, it has 

 been thought that the entire nucleus or 

 macula of the germinal vesicle has remained 

 undivided, and it has been supposed that it 

 has of itself constituted the germ of the 

 embryo-cell, which afterwards occupies the 

 centre of the first segmenting mass of the 

 yolk, and whose progeny by division exists as 

 nuclei in the interior of the successively in- 

 creasing segments of the cleaving germinal 

 portion of the yolk. A recent observation 

 by J. Muller seems to lead the way to a 

 different view of this phenomenon. He has 

 observed * in one of the Mollusca, the Ento- 

 choncha mirabilis, that the germinal vesicle 

 does not disappear or undergo a change at 

 the time of the maturation of the ovum, but 

 remains discernible as the foundation of the 

 clear embryonic -cell which occupies the centre 

 of the yolk mass when segmentation is about 

 to take place. Rernak f has been led, by his 

 observations on the Batrachian ovum, to 



* Archiv. der Physiol. 1852. Leydig in the same. 

 + Untersuch. iiber die Eutwickel. der Wir- 

 berthiere. 



