200 OVIPARA. 



gous to the allantoid of the M'ammalia. It is neither found in fishes nor 

 in the batrachians, which latter, when young, respire, like fishes, by 

 branchiae. 



fowls, for the apparatus for nutrition and respiration has different or distinct termi- 

 nations ; whereas in fowls and quadrupeds, all the vessels enter at one place. A 

 similar fact is observed in the ova of frogs, for the umbilical cord in the tadpole 

 goes to the head. 



' The egg of the serpent is nearly the same with that of the fish, and is enclosed 

 in a flexible membrane. The foetus is coiled up spirally within it, and the chorion 

 is vascular, as in the egg of the fowl. 



' The adder is a viviparous animal; its uterus is membranous, and divided, I find, 

 into eight or nine cells, each of which, in September, contains an ovum as large as 

 a chesnut. This consists of an exterior membrane, which encloses a foetus about 

 six inches long, and coiled up. About an inch from the tail, the umbilical cord 

 passes out, which consists of vessels that go to ramify on the exterior membrane, 

 which resembles the chorion of the sow. There is also a connexion with a vitellus, 

 which is as large as a hazel nut. 



' The coluber natrix is said, by Valmont-Bomare, to have a placenta and cord 

 within the egg, but this is contrary to the general structure of eggs ; most likely the 

 chorion has been taken for the placenta. The eggs of reptiles are often deposited in 

 packets, the eggs being glued together. 



' The egg of the turtle is as large as a hen's, and is enclosed in a covering like 

 parchment. It is deposited in the sand, and is hatched in about twenty-four days. 

 The egg of the alligator is similar in structure to that of the turtle: it is rather 

 larger than a goose's egg, and covered with a thin skin, so transparent, however, that 

 the foetus may be seen through it. 



' Those animals which are called oviparous, hatch their eggs out of the body, 

 either by sitting on them, as we see in fowls, or by exposing them to the heat of the 

 sun, as the turtle, crocodile, and many serpents. Oviparous fishes, which compre- 

 hend all those called osseous, expel their ova into the water, where they are fecun- 

 dated by the male, but without copulation. Many fishes leave the sea, and come up 

 file rivers to spawn. Others remain in the ocean; and the eggs, specifically lighter 

 than the water float on the surface. Many fishes attach them to marine plants, and 

 in some cases the ova are fixed to the body of the parent. The ova are covered 

 with a kind of mucus, which has been supposed to defend them from the water. 



' The ova of frogs, &c. are likewise fecundated and hatched out of the body. They 

 are enveloped in a glairy matter, which, perhaps, contributes to their increase; for 

 during incubation, the egg both enlarges and changes its shape. 



' Those animals which hatch their eggs within the body, are called ovo-viviparous, 

 such as cartilaginous fishes, as the shark, skate, torpedo, &c. The scorpion and 

 venomous serpents also belong to this class. Ovo-viviparous animals expel the 

 young fully formed, and therefore have been sometimes considered as having uteri 

 like quadrupeds, and a cord attached directly to it. Spallanzani at first supposed 

 that the foetus of the torpedo was attached directly to the uterus, but afterwards 

 found that it was contained in a distinct ovum. Experiences, p. 294. See also 

 Cuvier Legons d'Anat. Comparee, Tom. V. p. 132. The shark is said to have an 

 uterus like the bitch, and Belon says he saw a female delivered of eleven young at- 

 tached by a cord. Its mode of gestation most likely is similar to the torpedo. This 

 class expel their young often very quickly. A female syngnatus hyppocampus was 

 observed to expel at least a hundred in a very short time. 



' Analogous to ovo-viviparous animals, are those which receive the ova into cells 

 on the surface of the body, where they are hatched. This is well seen in the pipa, 

 a species of toad. Even the tadpoles are said to be metamorphosed in these cells. 

 The oppossum tribe has a modification of this gestation; for in them the foetus, 

 when very small, is expelled into a bag situated on the belly, and immediately at- 

 taches itself to a nipple. The utero-gestation of the oppossum of North America 

 lasts only from twenty to twenty-six days, and the embryo, when expelled, does not 

 exceed a grain. It remains in the sac about fifty days, and acquires the size of a 

 mouse. In other animals, as for instance the bat, the young, after birth, attach 

 themselves to the nipple, partly for the convenience of being transported or carried 

 about. 



