OVUM 



appearance: when, however, the vesicle has 

 been extracted from the yolk, I have gene- 

 rally found this appearance to be removed and 

 perfect sphericity restored. It would appear 

 also, from Vogt's observations in Alytes, that 

 this appearance is not constant : it may depend 

 on the viscidity of the contents, and the ex- 

 treme softness and thinness of the enclosing 

 membrane of the vesicle. 



It is only in the earliest stages of ovarian for- 

 mation that any appearance of distinct maculae, 

 such as they have been described in other 

 animals, is to be perceived ; for from a very 

 early period these spots or nuclei are already 

 very numerous. As the ova approach matu- 

 rity the contents of the germinal vesicle un- 

 dergo very considerable and rapid changes, 

 by which a number of corpuscles, some loose, 

 others aggregated, and subsequently delicate 

 cells, are formed, and completely fill the whole 

 cavity of the vesicle. 



The germinal vesicle is situated, in the 

 ripe ovarian ovum, nearer the upper than the 

 lower part of the yolk. When the egg has 

 been hardened by the re-agents already re- 

 ferred to, there can be perceived in the middle 

 of the upper surface, or exactly in the upper or 

 germinal pole of the yolk, a minute depression, 

 which was first noticed by Prevost and Du- 

 mas *, and which they, erroneously, according 

 to most of the observers who have followed 

 them, conceived to be connected with an aper- 

 ture or pore in the external membranes of the 

 ovum. Von Baer showed that this depres- 

 sion leads into a canal which extends from 

 the upper pole of the yolk, through the yolk- 

 substance, to the surface of the germinal 

 vesicle. The existence of this canal has been 

 fully established, and its situation well repre- 

 sented by Newport. The interval between 

 the upper surface of the yolk and the germinal 

 vesicle appears to become less as the ovum 

 approaches maturity. 



The vitelline membrane of the mature 

 ovarian ovum in the frog is thin and homoge- 

 neous. In the ova which have escaped from 

 the ovary into the abdominal cavity it is still 

 so thin, that they are very liable to be broken 

 by the slightest force applied unequally on 

 their surface; but in their descent through 

 the oviduct considerable consistence is given 

 by the addition of the layers of albumen to the 

 vitelline membrane. Besides the simple vitel- 

 line membrane, there appears to be a second 

 envelope formed within the albuminous de- 

 posit. Remakj", indeed, describes the vitelline 

 membrane itself as consisting of two layers, 

 besides the superadded membrane within the 

 albumen. 



Formation of the Ovum, and Changes in its 

 Progress. The ovary of the Batrachia is 

 peculiarly well adapted for making observa- 

 tions on the development of the ova, as the 

 stroma is in small quantity and transparent, 

 and as it contains at most seasons a considerable 



* 2 me Mem. sur la Generation, &c., in Annal. 

 des Scien. Nat 1824, torn. ii. p. 104. 

 t Loc. cit. p. 127. 



number of ova in different stages of their for- 

 mation and progress. If examined in the 

 autumn or in spring before pairing, there are 

 generally found three sets of ova ; one uniformly 

 large and dark-coloured, obviously belonging 

 to those which are about to be brought forth 

 in the ensuing breeding season ; another set, 

 also of uniform size, but less than the 

 first, and in which only a partial deposit of 

 colouring matter has taken place, probably 

 constitute the ova for the next season after the 

 first ; and, third, a number of ova of inferior 

 magnitude to either of the other sets, and of 

 most various sizes, down to the most minute, 

 which we may suppose to comprise those de- 

 stined for succeeding breeding seasons. It 

 seems probable that three seasons are neces- 

 sary for the full development of the ova in 

 the common frog and toad. 



The earliest ova are seen within the ovi- 

 capsules or ovisacs, in the delicate ovarian 

 stroma ; the more advanced are enclosed in 

 their pediculated capsules or calyces. 



The germinal vesicle is the part of the ovum 

 first distinctly recognisable; but so soon as it, 

 or any part of the ovum can be distinguished, 

 the delicate membrane of the ovisac or ova- 

 rian follicle is also seen surrounding it. 

 Leuckart* was never able to perceive a fol- 

 licle without there being already also an ovum 

 within it. It would appear, therefore, either 

 that the follicle and germinal vesicle arise 

 together, or that observations have not yet 

 determined which of them has the priority. 

 It has been stated by some, that in the very 

 earliest periods a single macula or nucleus may 

 be observed in the germinal vesicle f ; but it 

 is rare to find the germinal vesicle in this state, 

 and I have generally observed the macula, even 

 in the earliest stages, to be multiple, or to con- 

 sist of several maculae. Still it is undoubted 

 that, in the earliest period, there are fewer 

 maculae than at more advanced periods, and 

 that their number gradually increases. About 

 the time of maturation of the ovum the con- 

 tents of the germinal vesicle undergo further 

 changes, to which reference will hereafter be 

 made. 



From a very early period, though perhaps 

 not from the first, the germinal vesicle is sur- 

 rounded by a thick viscid substance, which 

 closely adheres to its surface. This substance 

 is at first remarkably clear, especially at its 

 outer part, where it has a hyaline appearance : 

 a little later it becomes gradually more and 

 more opaque, as if by the deposit in or mix- 

 ture with its clearer substance of fine mole- 

 cules or granules. This appears to be the 

 primitive yolk-substance ; which in these ani- 

 mals therefore, as inmost others, is ascertained 

 to consist of a clear basis or matrix, in which 

 the granular part is suspended. The out- 



* Loc. cit. 



t Mr. Newport describes the germinal vesicle of 

 the frog's ovum as nucleated, even when half-grown. 

 He also speaks of the corpuscles of the yolk sub- 

 stance as "nucleated cells" (1st Series, p. 176.) ; 

 but this is quite inconsistent with the statements 

 of most other observers. 



