COMPARATIVE ANATOMY. 



"masses of an apparently coagulated sedi- 

 ment in various forms and number, which 

 has been long 1 known by the singular 

 name of the horse-venom, or hippomanes. 

 Some orders and genera of mammalia 

 resemble the human subject in having no 

 allantois, as the quadrumana and the 

 hedge-hog ; nay, in the latter animal, the 

 urinary bladder has no trace whatever of 

 urachus. 



0:y THE IXCTIBATED EGG. 



The various vital processes of nutrition 

 and formation, which are carried on in the 

 foetus of the mammalia while in its mo- 

 ther's body, and by means of the most 

 intimate connection with the parent, are 

 effected in the incubated chick by its 

 own powers, quite independently of the 

 mother, and without any extraneous as- 

 sistance, except that of the atmospheric 

 air and a certain degree of warmth. 



The egg is covered within the shell, 

 by a white and firm membrane (mem- 

 branaalbuminis) which contains no blood- 

 vessels. The twolayersofthismembrane, 

 which in other parts adhere closely to 

 each other, leave at the large end a 

 space which is filled with atmospheric 

 air. 



This membrane includes the two 

 whites of the egg, each of which is sur- 

 rounded by a delicate membrane. The 

 external of these is the most fluid and 

 transparent ; the inner one thicker and 

 more opaque ; they may be separated in 

 eggs which are boiled hard. 



The internal white surrounds the yolk, 

 which is contained in a peculiar mem- 

 brane called the yolk-bag. From each 

 end of this proceeds a white knotty body, 

 which terminates in a flocculent extre- 

 mity in the albumen. These are called 

 the chalazx, or grandines. 



A small round milk-white spot, called 

 the tread of the cock (cicatricula, or ma- 

 cula), is formed on the surface of the 

 yolk-bag. It is surrounded by one or 

 more whitish concentric circles (halones, 

 or circuli), the use of which, as well as 

 that of the cicatricula itself, and of the 

 chalaz<e, is not yet ascertained. 



We now proceed to notice the wonder- 

 ful successive changes which go on during 

 the incubation of the egg, and the meta- 

 morphoses which are observed, both in the 

 general form of the chick and in particu- 

 lar viscera. The periods of these changes 

 will be set down from the hen, as afford- 

 ing the most familiar example. It will be 

 best to give, first, a cursory chronological 



VOL. Ill, 



view of the whole process, and then to 

 make a few remarks on some of the most 

 important parts of the subject. 



A small shining spot, of an elongated 

 form, with rounded extremities, but nar- 

 rowest in the middle, is perceived at the 

 end of the first day, not in nor upon the 

 cicatricula, but very near that part on the 

 yolk-bag (nidus pulli ; colliquamentum ; 

 areola pellucida.) This may be said to 

 appear before-hand, as the abode of the 

 chick which is to follow. 



No trace of the latter can be discerned 

 before the beginning of the second day; 

 and then it has an i n curv ate d form, resem- 

 bling a gelatinous filament with large ex- 

 tremities, very closely surrounded by the 

 amnion, which at first can scarcely be dis- 

 tinguished from it. 



About this time the halones enlarge 

 their circles, but they soon after disappear 

 entirely, as well as the cicatricula. 



The first appearance of red blood is 

 discerned on the surface of the yolk-bag 

 towards the end of the second day. A 

 series of points is observed which form 

 grooves ; and these, closing, constitute 

 vessels, the trunks of which become con- 

 nected to the chick. The vascular sur- 

 face itself is called figura venosa, or area 

 vasculosa ; and the vessel, by which its 

 margin is defined, vena terminalis. The 

 trunk of all the veins joins the vena por- 

 tae ; while the arteries, which ramify on 

 the yolk-bag, arise from the mesenteric 

 artery of the chick. 



On the commencement of the third day, 

 the newly -formed heart(the primary organ 

 of the circulating process which now com- 

 mences) is discerned by means of its triple 

 pulsation,andconstitutesa threefold punc- 

 tum saliens. Some parts of the incubated 

 chicken are destined to undergo succes- 

 sive alterations in their form ; and this 

 holds good of the heart in particular. In 

 its first formation it resembles a tortuous 

 canal, and consists of three dilatations ly- 

 ing close together, and arranged in a tri- 

 angle. One of these, which is properly 

 the right, is then the common auricle ; 

 the other is the only ventricle, but after- 

 wards the left ; and the third is the dilat- 

 ed part of the aorta (bulbus aortae.) 



About the same time, the spine, which 

 was originally extended in a straight line, 

 becomes incujvated; and the distinction of 

 the vertebrae is very plain. The eyes may 

 be distinguished by their black pigment, 

 and comparatirely immense size ; and they 

 are afterwards remarkable, in conse- 

 quence of a peculiar slit in the lower 

 part of the iris. 



TT n 



