OVIPARA. 199 



ill addition, in the egg, a highly vascular membrane, which appears to be 

 suhservient to respiration ; it is connected with the bladder, and is analo- 



and comes presently to cover the inner surface of the memhrane of the shell. It h 

 the chorion, and has numerous vessels ramifjang on it, like the chorion of the sow, 

 and connected in like manner with the foetus. The blood of the umbilical artery is 

 dark coloured, that of the vein bright As incubation advances, the amnion enlarges, 

 and comes in contact every where with the chorion. The albumen is all consumed, 

 being taken into the vitellus, which is in a great measure absorbed; and what re- 

 mains is taken, together with the sac, into the abdomen of the chick, and the parie- 

 tes close over it. On the 21st day, the chick breaks the shell and escapes. By in- 

 creasing or diminishing the temperature within a certain extent, the process may bo 

 somewhat accelerated or retarded. The eggs of large birds require a longer time to 

 be hatched; those of the ostrich, for example, take six weeks. 



' Hence it appears, that the vitellus and albumen contribute to the increment of 

 the foetus, whilst the exterior membranes act as lungs, the air being transmitted 

 tlii-ough the pores of the shell. 



' Sir Everard Home has lately investigated more minutely, by means of the 

 microscope, the structure of the egg. The yolk is enclosed in two layers of mem- 

 brane, the outermost very thin and delicate; the internal one, on the contrary, thick 

 and spongy, with a small aperture, or deficiency, at one spot. Here the gelatinous 

 molecule, from which the embryo is to be formed, is found placed on the surface of 

 the yolk. It is not quite so large as the opening in the membrane, and, therefore, 

 seems to be surrounded by a margin, or an areola. In its passage along the oviduct, 

 the yolk comes to be surrounded by the albumen, which presently is covered with a 

 very fine membrane. During this descent are also formed the chalazes, which ter- 

 minate in that double membrane which is added when the egg has reached the lower 

 end of the oviduct, and which is distinct from, and exterior to the fine membrane 

 already noticed. In four hours after the commencement of incubation, the outer 

 edge of the areola had become enlarged, and that part next the molecule darker. 

 One end of the molecule appeared like a white line, which is the rudiment of the 

 embryo. In eight hours the white line had extended, and rudiments of the brain 

 and spinal marrow could be discerned, surrounded by a membrane which afterwards 

 becomes the amnion. The areola had extended, and the surface beyond the line 

 which had formed its boimdary had acquired the consistence of a membrane, and 

 was circumscribed by a distinct line. This he calls the outer areola. In thirty-six 

 hours, a vesicle had begun to protrude under the inner areola, apparently at the ter- 

 mination of the spinal marrow. In sixty hours, auricles and ventricles were seen, 

 the former filled with red blood; a trunk from the left ventricle gives off two large 

 vessels which send branches over the whole areolar membrane. The vesicle some- 

 what enlarged. In three days, the vesicle, still more enlarged, had forced its way 

 through the external covering of the yolk, whereby a part of the albumen was ad- 

 mitted to mix with the yolk. In four days, the vesicle was still larger, and its vessels 

 contained red blood. On the fifth, it was of great size, very vascular, and its cavity 

 contained a fluid. On the sixth, it had expanded, like a double nightcap, over the 

 yolk, and its coverings were beginning to enclose the embryo. This vesicle. Sir 

 Everard, in another place, compares to the bag which, in the human subject, and in 

 quadrupeds, is to become the urinary bladder. This increases with such rapidity 

 that it bursts the amnion, and the arteries lying on its two sides are carried directly to 

 the chorion, and there the placenta is formed in the empty space between the two 

 edges of the chorion. Phil. Trans. 1823, Part II. p. 339. Vide also a paper by 

 Dutrochet on the envelopes of the foetus, in the Bull, de la Soc. de Med. 1819, No. 

 8. And :\Iem. de la Societe d'Emulation. Tom. VIII. pp. 3, 760. 



' The eggs of fishes have a general resemblance to those of fowls, and consist of 

 a vitellus and albumen, with their membranes; but in place of being furnished with 

 a shell they have a tough, or sometimes a horny covering; and some, as those of the 

 shark, torpedo, &c. are quadrangular in shape. The yolk is connected to the intes- 

 tines of the foetus, and its membrane is very vascular. As in fowls, so in fishes, it 

 is ultimately enclosed within the abdomen of the young. In the skate, mmierous 

 blood vessels are fonned in the albumen, which supply the place of gills, and are 

 supposed by Dr. Monro, to be afterwards covered and converted into gills. The two 

 functions of a placenta, then, are still more distinctly fulfilled here than even in 



