COMPARATIVE ANATOMY. 



numerous (more than 200,000 in the carp ) ; 

 and terminate by a common opening be- 

 hind the anus. 



The immense number of ova contained 

 in the ovaria of fishes accounts to us sa- 

 tisfactorily for the astonishing 1 multitudes 

 in which some species are formed. In a 

 perch weighing- one pound two ounces, 

 there were 69,216 ova in the ovarium ; in 

 a mackarel of one'pound three ounces, 

 129,200 ; in a carp of eighteen inches, Pe- 

 tit found 342,144 ; and in a sturgeon of 

 one hundred and sixty pounds, there was 

 the enormous number of 1,467,500. 



EMBRYO OF THE MAMMALIA. 



The mode of connexion of the pregnant 

 uterus with the membranes of the ovum, 

 and thereby with the embryo itself, dis- 

 play three chief differences in the vari- 

 ous mammalia. 



Either the whole external surface of the 

 ovum adheres to the cavity of the uterus, 

 or the connection is effected by means of 

 a simple placenta, or by more numerous 

 small placentae (cotyledons.) 



The first kind of structure is observed 

 in the sow, and is still more manifest in 

 the mare. In the latter case, the external 

 membrane of the ovum, the chorion, may 

 be said to form a bag-like placenta. Nu- 

 merous and large branches of the umbilical 

 vessels ramify through it, particularly in 

 the latter half of the period of pregnancy; 

 and its external surface is covered with 

 innumerable flocculent papillae, which 

 connect it to the inside of the uterus. 



In those animals of this class, where the 

 embryo is nourished by means of a pla- 

 centa, remarkable varieties occur in the 

 several species ; sometimes in the form 

 and successive changes of the parts ; some- 

 times in the structure of the organ, as be- 

 ing more simple or complicated. 



In most of the digitated mammalia, as 

 well as in the quadrumana, the placenta 

 has a roundish form ; yet it consists some- 

 times of two halves lying near together ; 

 and in the dog, cat, martin, &c. it resem- 

 bles a belt (cingulum or zona.) Its form 

 in the pole-cat holds the middle between 

 these two structures, as there are two 

 round masses joined by an intervening 

 narrower portion 



The placenta of the bisulca is divided 

 into numerous cotyledons, the structure of 

 which is very interesting, as it elucidates 

 the whole physiology of this organ. The 

 parts designated by this appellation are 

 certain fleshy excrescences (glandulae 

 uterinx) produced from the surface of the 



impregnated uterus, and having a corres- 

 ponding number of flocculent fasciculi of 

 blood-vessels (carunculae) which grow 

 from the external surface of the chorion 

 implanted in them. Thus the uterine and 

 fetal portions of the placenta are manifest- 

 ly distinct from each other, and are easily 

 separable as ; the fcctus advances to matu- 

 rity. The latter only are discharged with 

 the after-birth, while the former, or the 

 cotyledons, gradually disappear from the 

 surface of the uterus after it has parted 

 with its contents. The number and form 

 of these excrescences vary in the different 

 genera and species. In the sheep and cow 

 they sometimes amount to a hundred. In 

 the former animal and the goat they are, 

 as the name implies, concave eminences ; 

 while, on the contrary, in the cow, deer, 

 &c. their surface is rounded or convex. 



The trunks of the veins which pass from 

 the placenta or carunculse, and of the ar- 

 teries which proceed towards these parts, 

 are united in the umbilical chord, which is 

 longer in the human embryo than in any 

 other animal. 



In the foal, as in the child, the chord 

 possesses a single umbilical vein ; whilst 

 most other quadrupeds have two, which 

 unite, however, into a common trunk near 

 the body of the foetus, or just within it. 



The amnion, or innermost of the two 

 membranes of the ovum, \vhich belongs to 

 the pregnant woman, as well as to the 

 mammalia, is distinguished in some of the 

 latter, as for instance in the cow, by its 

 numerous blood-vessels; while, on the con- 

 trary, in the human subject it possesses 

 no discernible vascular ramification. 



Between the chorion and amnion there 

 is a part found in most pregnant quadru- 

 peds, and even in the cetacea, which does 

 not belong to the human ovum, viz. the 

 allantois, or urinary membrane. The lat- 

 ter name is derived from the connection 

 which this part has, by means of the ura- 

 chus, with the urinary bladder of the 

 foetus : whence the watery fluid which it 

 contains has been regarded as the urine 

 of the animal. The term allantois 

 has arisen from the sausage-like form 

 which the part possesses in the bisulca 

 and the pig ; although this shape is not 

 found in several other genera and spe- 

 cies ; thus, in the hare, rabbit, guinea-pig, 

 &c. it resembles a small flask ; and it is 

 oval in the pole-cat. It covers the whole 

 internal surface of the chdrion in the 

 solidungula, and therefore incloses the 

 foal with its amnion. It contains, most 

 frequently in these animals (although 

 not rarely in the cow,) larger or smaller 



