OVUM. 



51 



sule of similar consistence and structure, but 

 of a simple elongated ellypsoidal shape, and in 

 place of four terminal angular tubes, a number 

 of trumpet-shaped tubufar processes project- 

 ing from the middle of the two ends, which 

 probably serve the same purposes as the 

 differently shaped appendages of the ova of 

 the shark and skate. 



Fig. 33. 



T) 



External form of ova of Oviparous Cartilaginous 

 Fishes. 



A. Ovum of the common skate fish, a portion re- 

 moved from one side of the coriaceous envelope to 

 show the yolk floating in the white : one-third the 

 natural size. 



B. Ovum of the shark, squalus catulus, also 

 opened : half the natural size. 



c. Ovum of the Myxine glutinosa, entire : natural 

 size. 



D. Enlarged view of one of the 2 30 tubular 

 funnel-shaped processes from the same ovum; the 

 attached end is at D. 



In birds, it is well known that the yolk and 

 germ, with their enclosing vitelline membrane, 

 are produced in the ovary, while the albumen, 

 chalazae, membrane, and shell are more rapidly 

 formed and are added during the passage of the 

 egg through the oviduct. It is by an entirely 

 similar process that these accessory parts are 

 formed in the scaly reptiles, the eggs of which 

 agree with those of birds in the most essential 

 points. The albumen, however, is generally 

 in less quantity and softer, and the twisted 

 chalazae have not been observed. The mem- 

 brane which immediately covers the albumen 

 has the same structure'as that of the bird's 

 egg; and the calcareous shell, when it exists, 

 as in turtles and crocodiles, is more porous 

 and thinner. 



In cartilaginous fishes there is also a glairy 

 albumen investing the yolk, and secreted from 

 the oviduct. In most animals of the second 



group, or with the large-yolked ova, the 

 vitelline substance consists almost entirely of 

 oily and albuminous matter enclosed in or- 

 ganised cells, the nature of which differs, as 

 previously explained, in the vicinity of the 

 germ and in the other parts of the yolk ; this 

 substance contains, besides, the peculiar co- 

 louring matter which has given the name to 

 this part of the egg. In all of them a cicatri- 

 cula exists, which is the seat of the germinal 

 vesicle, and of the first formation of the 

 rudiments of the embryo. 



The ovum of the frog, when newly expelled 

 from the oviduct of the parent, consists of 

 the yolk-ball, closely surrounded by a tough 

 layer of peculiar albuminous matter deposited 

 on it in the course of its passage through the 

 oviduct. This substance has the property 

 of imbibing a large, but yet a limited, quantity 

 of water whenever it is immersed in it ; and 

 thus, within a short time after the expulsion 

 of the egg from the female, the external sub- 

 stance has assumed a gelatinous consistence, 

 ana has enlarged to such an extent as to be 

 on every side equal in thickness to the dia- 

 meter of the dark-coloured yolk within. I 

 shall have occasion afterwards to state more 

 particularly the important relation which 

 subsists between this process of imbibition 

 and the action of the spermatic substance in 

 fecundation. In the common frog, the ova 

 are thus united in large masses, floating in the 

 water of stagnant pools or rivulets : in the 

 common toad, they are united in long cords, 

 which become entangled among aquatic plants. 



In the newts, the external covering of the 

 ovum is membranous, homogeneous and 

 transparent, and of an elongated oval shape, 

 and there is merely fluid intervening between 

 it and the spherical yolk and its membrane ; 

 but when the ova are deposited by the parent 

 in the folded leaves of water-plants or other 

 situations, a small quantity of a peculiar glu- 

 tinous matter, not readily acted on by water, 

 is excreted along with the ova, which serves 

 to fix the ova in a suitable place during the 

 development of the young.* Various ex- 

 amples of a similar kind occur among the 

 oviparous animals of the invertebrata, more 

 especially among insects and mollusca, when 

 the ova are destined to remain exposed, and 

 require protection during a considerable time 

 before development takes place. 



In batrachia the yolk is variously coloured 

 in different species : thus, in the common frog, 

 toad, and some others, the surface or ger- 

 minal part of the yolk is of a black or dark- 

 brown colour, owing to a deposit of pigment 

 granules in the cells of the germinal layer, 

 while the remainder of the yolk internally is 

 grey. In some other batrachia the colour 

 is light brown. In the larger water-newt, or 

 triton, the yolk is of a brilliant light yellow ; 

 while in the smaller one, or lissotriton, it is 



* See the interesting description of this process 

 by Rusconi, in Amours des Salamandres Aquatiques ; 

 Milan, 4to. 1821. I have often confirmed his ob- 

 servations on this process in ponds, and with animalg 

 kept in vessels in the house. 



E 2 



