OVUM. 



55 



and serpents, and in the batrachia, this organ 

 is hollow, and the capsules in which the 

 ovules are formed burst in dehiscence into 

 an internal cavity, whence the ovules escape 

 into the abdomen by the rupture or open- 

 ing of the sack of the oviduct, generally 

 at one, but sometimes, as in the frog, at a 

 greater number of places. In the higher 

 animals, in which the ovules escape from 

 the external surface of the ovary, their en- 

 trance into the oviduct is in general secured 

 by the temporary apposition of the dilated 

 upper end or infundibulum of the oviduct to 

 the ovary, or the capsule containing a ripe 

 ovule ; in the other animals, in which the 

 ova come from the interior of the hollow ovary, 

 the apposition of the oviduct does not ap- 



Fig. 38. 



Common adder, in which the ova have descended 

 to occupy both oviducts, five in the right, and three 

 in the left : the infundibulum is shown in each ovi- 

 duct; a' a, the right and left ovaries, each forming 

 a sac, opening anteriorly near the infundibulum for 

 the discharge of the ova, which, when ripe, fall 

 into the interior of the sac, and thence pass into the 

 oviduct. 



pear to be so direct, and there are various 

 other means by which the ova, when they have 

 escaped into the abdominal cavity, reach the 

 open extremity of the oviduct. 



It is in the class of fishes that the transi- 

 tion occurs from the higher to the lower type 

 of organisation of the ovaries and oviducts. 

 In all of them the ovules are formed in ova- 

 rian follicles, and escape by dehiscence from 

 these follicles ; but there are several modifi- 

 cations of the relation between the oviduct 

 and ovary among them. 1st. In the sharks 

 and rays, as already stated, the arrangement 

 is nearly similar to that existing in higher ani- 

 mals. The ova, which are of large size, come to 

 maturity singly, or in small numbers at once : 

 on being discharged externally from the ovarian 

 capsules, they pass into the oviduct, and there 

 receive a considerable addition from this 

 organ. The majority of them, as previously 

 stated, are oviparous, and in them a hard 

 covering is formed by a peculiar glandular 

 organ connected with the oviduct ; in a few 

 which are ovoviviparous, as the common dog- 

 fish, torpedo, &c., the external covering of 

 the ovum is membranous and soft. 2nd. In 

 the sturgeon and in the lamprey the oviduct 

 is very short ; still, as it opens superiorly into 

 the abdominal cavity, the relation may be 

 considered the same as in the previous ex- 

 amples. 3rd. In the genus salmo and in 



Fig. 39. 



B 



Ovaries and oviduct of an osseous Fish. 



A. Sketch of the two largely developed saccu- 

 lated ovaries of an osseous fish, with the short ovi- 

 ducts proceeding from near their posterior ex- 

 tremities. 



B. Diagrammatic section of a portion of the ova- 

 rian sac, showing two of the ovarian plates, from 

 which the developed ova hang in small pediculated 

 vesicles or ovisacs. 



E 4 



