OVUM. 



[123] 



or less of a minutely tuberculated structure 

 on its external surface. The ovum becomes 

 of a regular short oval or nearly spherical 

 form. If fecundation shall have" occurred, 

 the embryonic vesicle or cell makes its ap- 

 pearance, and the phenomena of segmentation 

 follow in rapid succession. 



Fig. 88*. 



Formation and fecundation of the ova of Nematoid 

 Worms. (According to Meissner.) 



a. A portion of the ovarian axis and early ova 

 attached to it from the ovarian tube of Strongylus 

 armatus. The axis column occupies the centre of 

 the tube, and the ova are suspended to it by 

 pedicles, supposed by Meissner to form micropyle 

 apertures when they are detached. 



b. View given by Meissner of a set of the nearly 

 ripe ova of Ascaris mystax, which he conceives are 

 thus connected by pedicles to a central axis. 



c. Two mature" ova of the same surrounded and 

 in part penetrated by spermatic corpuscles. At the 

 narrow angles of these ova a spermatozoon is seen 

 passing into the interior by what Meissner has 

 regarded as a micropyle formed by the detached 

 pedicle. In the ovum to the right a spermatic cor- 

 puscle is seen in the vitelline substance. The 

 existence of such a micropyle aperture and pedicu- 

 lated attachment of the ova in the Ascarides 1 re- 

 gard as doubtful. 



In others of the Nematoid Worms and more 

 especially in Strongylus and the Gordiacei, it 

 would appear from the researches of Meiss- 

 ner, that the first germs of ova which take 

 origin in the uppermost part of the ovarian 

 tube multiply by an endogenous production, 

 and that in this manner groups or bunches of 

 the primitive ova are produced which are con- 

 nected together by pedicles arising from the 



Fig. 89*. 



Formation of ova and fecundation in Gordius Sub- 

 bifurcus. (From Meissner.) 



a. A small portion of the ovarian tube with 

 groups of the ova partly within and partly escaping 

 from it. 



b. Three of the mature ova from the lower part 

 of the oviduct surrounded by the spermatozoa. The 

 ova are now isolated, and the pedicle of each is 

 open, and is regarded by Meissner as a micropyle, 

 by which spermatozoa, as represented in two of 

 them, enter the ova. The germinal vesicle is still 

 to be seen. 



elongated membrane of the original germ-cell 

 which remains as a covering of the whole. A 

 certain number of these ova make progress in 

 development while others probably become 

 abortive. As the ova enlarge they are more 

 spread out in the tube and take something of 

 the spiral disposition which exists in the Asca- 

 rides, but with this difference, as already noted, 

 that the various ova remain connected to- 

 gether by the attachment of their pedicles to 

 a central axis or stem running down the 

 middle of the ovarian tube. On the subse- 

 quent detachment of the ova by the break- 

 ing of these pedicles, according to Meissner, 

 a micropyle aperture is formed in each 

 ovum for the admission of the spermatozoa. 

 The accompanying drawings from Meissner's 

 Memoir will give a sufficiently clear idea of 

 his views on this subject. 



The ova of the nematoid worms constitute 

 a marked example of the simpler kind of ovum 

 in which the formative yolk is present, and 



