OVUM. 



LIOI] 



Fig. 68*. 



w^W^FS^f'iiW^ ..e 



~> vi- :\',:.'.';.-'.'.'.'.'!-'.:\^:.'~.'> :';'.'-;'':?'' ''".: ! . .'.-.'-, '-> ' 



: . .';:',"'''. '--" r'\ - ' ; ..'.'''' '..'" -. ; ' --'.:"..';'.'> '- '.':': 

 ':-f^'-i-' : '':''''..''."'' ':' ':' ' '.. :':^.[- ''':. " . -', p% !.< 



Micropyle of the ovum in Osseous Fiskes. 



A. Enlarged view of a quadrangular portion of the 

 surface of the mature ovarian egg of the Stickleback 

 containing the micropyle from above. In the outer 

 part of this figure the'general dotted appearance of 

 the membrane is seen, and here and there the pedicu- 

 luted flask-like processes attached to the membrane 

 in this fish in the vicinity of the micropyle ; the 

 radiated shading represents the appearance of the 

 funnel-shaped depression leading to the aperture of 

 the micropyle, which is seen in the centre of the 

 space it encloses. 



B. Transverse section of the dotted membrane 

 and funnel of the micropyle of the same egg some- 

 what more enlarged, seen in profile ; the aperture 

 of the micropyle is seen towards the point of the 

 funnel. This view is semidiagrammatic, and the 

 line canals passing through the membrane are re- 

 presented fewer and wider than they are in nature. 



The diameter of the whole ovum was about /' ; 

 the thickness of the external membrane T J gg " ; the 

 width of the base of the funnel about T i g " ; the 

 depth of the funnel 5 lg" ; the diameter of the micro- 

 pyle aperture at the apex 3^5". 



c. Small portion of the membrane at the apex of 

 the funnel containing the aperture of the micropyle 

 pressed flat, magnified 500 diameters; from the 

 trout's egg. 



D. A similar portion of the membrane magnified 

 1000 diameters. The lumen of the canals is seen, and 

 an indication of hexagonal division of the spaces 

 between them, represented somewhat too distinctly 

 in the figure. 



the existence of the micropyle in the ova o* 

 several fishes ; and though I have not yet 

 been so fortunate as to perceive the sperma- 

 tozoa actually passing into the ovum through 

 this aperture, the accuracy of Ransom's obser- 

 vations on this as well as on other points leave 

 little doubt as to the fact stated by him. 



The micropyle in the Gasterosteus, as de- 

 scribed by Ransom and observed by myself, 

 is a considerable funnel-shaped depression in 

 the outer membrane, which projects inwards 

 on the granular substance of the yolk, so as 

 to indent this layer to some depth, and pro- 

 bably to reach near to the germinal vesicle, 

 which lies imbedded within the germinal layer. 

 The inner narrow end of the funnel terminates 

 in a distinct rounded or elliptical mark, with a 

 fine but distinct line bounding it, which has 

 all the appearance of a foramen, and which is 

 either an open passage or one which is closed 

 only by an extremely delicate structure. 

 The funnel-shaped depression leading to the 

 micropyle may be easily seen on the surface 

 of the egg of the salmon or trout when slightly 

 dried of the adhering moisture, and is of such 

 a size that it may be perceived with the naked 

 eye or with a lens of low magnifying power. 

 In order to perceive the micropyle itself, how- 

 ever, or pore 'in the point of the funnel, it is 

 necessary to remove from the egg that portion 

 of the dotted shell membrane containing the 

 funnel ; and having freed it from the adherent 

 granules of the yolk-substance by careful wash- 

 ing, for which Ransom has recommended a 

 solution of acetate of potash, this part of it 

 may be viewed under pressure with great ease 

 with a magnifying power of 200 or 300 diame- 

 ters. The porous structure of the membrane 

 is then seen to continue very nearly up to the 

 margin of the micropyle. This last has a 

 diameter of from -soW' to Woo"- The ap- 

 pearance of a double outline surrounding the 

 micropyle proceeds from the circumstance 

 that, in looking through the funnel we see 

 at once two portions of the narrowing wall 

 of the passage of different widths. 



In Ransom's experiments, very soon after 

 spermatic fluid was placed in the water round 

 the ovum of the Stickleback, several of the 

 spermatozoa were perceived to pass in at the 

 micropyle ; and immediately upon this water 

 was imbibed, and the space named the respira- 

 tory chamber was formed between the yolk 

 surface and the external membrane ; a change 

 which in this fish did not take place in the 

 unfecundated ova, but which in some others 

 occurs without impregnation. It is from 

 this fact apparently that Ransom is inclined 

 to the opinion that the micropyle may be 

 closed by a very delicate membrane, which in 

 fecundation is removed or broken through by 

 the entrance of the spermatozoa; but with 

 regard to this point there is still some uncer- 

 tainty. The germinal vesicle previous to its 

 disappearance is imbedded below the super- 

 ficial layer of yolk-substance in a stratum of 

 granular matter ; and Ransom conceives that 

 at the time of the rupture of the vesicle, this 

 srraiuilar matter being mingled with the con- 

 til 3] 



