663 



REPRODUCTION IN PLANTS AND ANIMALS. 



REPRODUCTION IN PLANTS AND ANIMALS. 



870 



tozoon did not produce fecundation, but that the penetration of 

 several were required for this purpose. .. 



In the human female the ova are brought from the ovaries along 

 the Fallopian tube into an organ called the uterus. It grows rapidly 

 after reaching the uterus ; it at firat consists of two sacs, one inclosing 

 the other, and the inner containing a liquid. When it is about half a 

 line in diameter a new element becomes visible in it ; a round, opaque, 

 granular disc is seen, with a dark spot in its centre, upon the surface 

 of the internal globule or sac. This spot, which is seen either on or 

 through the inner membrane of the ovum, corresponds with the 

 cicatricula of the egg, and is the first rudiment of the foetus. 



In birds the cicatricula, or germ-spot, lies upon the surface of the 

 yolk : soon after the commencement of incubation it expands and 

 separates into two layers ; the outer is called by Pander the serous 

 layer, and subsequently forms the osseous, nervous, muscular, and 

 tegumentary systems of the body ; the inner, which is in contact with 

 the yolk, is called the mucous, which (together with a third developed 

 between the two others, and named the vascular layer) appears to give 

 rise, by the changes which it undergoes, to the intestinal, respiratory, 

 vascular, and glandular systems. The mucous layer of the germinal 

 membrane gradually expands over the yolk, till it nearly incloses it in 

 a sac, which towards the body of the chick contracts into an oblong 

 canal, which extends the whole length of the embryo, and becomes the 

 future alimentary tube. The sac containing the yolk, and communi- 

 cating with the intestines, is called the intestinal vesicle, or yolk-bag, 

 and towards the close of incubation is drawn into the belly of the 

 chick, and its contents are used as nourishment. The lower end of 

 the alimentary canal (the cloaca of birds) shoots out into a sac which 

 is termed the allantois, or al Ian to id membrane. After a time arteries 

 and veins are seen ramifying upon this sac, which protrudes more and 

 more out of the body of the chick, till at length it forms a double bag, 

 laid immediately under the membrane of the shell. On this sac the 

 blood-vessels are so distributed that their contents are influenced by 

 the atmosphere through the porous e?g-shell and its membrane, and 

 thus a true respiratory organ is established. 



The original structure of the ovum, and the early development of 

 the embryo, in Mammalia, appear to be much the same as in the egg 

 of a bird ; though there are some characteristic differences. When a 

 human ovum of any magnitude is examined, the embryo is seen sus- 

 pended in a loose bag filled with fluid, called the amnios, which is a 

 shut sac : this sac U the outermost product of the serous layer of the 

 germinal membrane ; for its formation a membrane is reflected from 

 the sides and extremities of the embryo (the reflection, according to 

 Velpeau, not commencing before the twelfth day), so as to inclose a 

 space behind it. As the walls of the trunk close in front, the circle 

 at which the amnios is attached to the body of the embryo gradually 

 contracts, till at length it is limited to the edge of the umbilical open- 

 ing ; it then invests the umbilical cord, and spreads out from its 

 placental extremity into an ample sac filled with fluid, in which the 

 foetus floats. The mucous layer of the germinal membrane in Mam- 

 malia is supposed from analogy to form a sac, as in birds, containing 

 a yolk, or substance subservient to the nourishment of the foetus in 

 its early stage. Whether this view of its formation and use be correct 

 or not only rests on analogy ; but in the early part of gestation a 

 small sac or bladder, which from its being filled with a whitish fluid 

 has been called the vesicula alba, may be found on the placenta, at or 

 near the extremity of the umbilical cord, and exterior to the amnios ; 

 from this sac a fine tube may be traced along the cord to the navel, 

 and in some animals it has been seen communicating with the intestinal 

 canal. This tube becomes obliterated so early (Velpeau says in the 

 sixth week of gestation) that its communication with the intestines 

 was long undetected, though the sac was known to the older anato- 

 mists. The intestinal vesicle finally differs in Mammalia and birds in 

 this circumstance, that in the former it is not drawn iuto the body of 

 the foetus, but remains without between the membranes, and gradually 

 wasting becomes obliterated by the third month. The duct of the 

 umbilical vesicle is accompanied along the cord by an artery and vein, 

 which are called the ompbalo-mesenteric vessels ; the artery com- 

 municates with the superior mesenteric, and the vein with the vena 

 portal. The allantois exists in all mammals as well as in birds, though 

 its use in the former, which are furnished with a placenta, is not 

 obvious. In some animals, as in man, it becomes obliterated at a very 

 early period, as soon as the sixth week, but in others, as the C'arnivora, 

 lie., it attains a large size, and continues during the whole period of 

 foetal existence. In Mammalia it communicates with the fundus of 

 the bladder, and the remains of the duct by which it is connected is 

 denominated the urachus. The channel of communication between 

 the allantois and the bladder, or cloaca (in birds), at first is short, so 

 that the sac lies directly against the body of the embryo, but it after- 

 wards becomes elongated, like the corresponding duct of the umbilical 

 vesicle. 



In man, after impregnation has taken place, a spongy membrane is 

 formeJ on the inner surface of the uterus by an exudation of lymph. 

 This membrane, called decidua, lines the whole of the uterus before 

 the descent of the ovum ; but when this passes down through the 

 Fallopian tube it gradually pushes the deciduous membrane before it, 

 inverting one portion of it which surrounds the ovum, and is called 

 the decidua reflexa; thi grows with the ovum till it fills the cavity 



of the uterus, and comes in contact with the other portion called tlie 

 decidua vera, lining the walls of the uterus. 



The point at which the decidua is reflected upon the ovum is where 

 the placenta is fixed to the uterus. The ovum has two proper mem- 

 branes, the amnios, which we have described, internally, and an outer 

 membrane, which is called the chorion ; thia latter membrane in man, 

 during the first two months of pregnancy, has a shaggy external 

 surface, being covered with vascular villi, which become united with 

 the membrana decidua, which is also thick and vascular. This 

 thickening and vascularity of both these membranes gradually 

 diminishes, and becomes concentrated on one part, usually towards 

 the fundus of the uterus ; this thickened part is called the placenta. 

 In ruminating animals the thickening and vascularity of the chorion 

 is confined to a number of circular and spongy elevations varying in 

 number from thirty to one hundred, which are called cotyledons. 

 These vascular processes dip in between corresponding processes 

 attached to the uterus of the mother, which are called maternal cotyle- 

 dons, the surface of which is supplied with numerous vessels derived 

 from the uterine arteries and veins. The result of this arrangement 

 is that a large vascular surface of the maternal system is applied to an 

 equally extensive one of the foetus, and though there is no direct 

 communication between the arteries and veins, we must suppose that 

 nourishment is imbibed from the vessels of the mother by those of 

 the foetus through the flue intervening membranes by which they are 

 separated. In man the relation between the maternal and foetal 

 systems is not so clearly understood as in the preceding instance. In 

 the human subject the placenta is a spongy vascular mass like a cake, 

 from six to eight inches in diameter, about an inch thick iu the middle, 

 and two or three lines at the circumference. It adheres by one 

 surface to the uterus, and by the other is connected with the footus 

 by means of the umbilical cord. The uterine surface is lobulated, 

 and is connected with the uterus by blood-vessels. The foetal surface 

 is covered by the chorion and amuios, and presents the ramifications 

 of the umbilical vessels, which consist of two arteries and a vein. The 

 radicles of these vessels communicate with each other, but no commu- 

 nication has ever been shown to exist between them and the utero- 

 placentine vessels ; for if we inject from the umbilical arteries we find 

 that the placenta is rendered turgid, and that vessels are found filled 

 in every part of it, but between their ramifications there will remain 

 an uuinjected substance, and the uterine surface will not be injected, 

 for the fcetal vessels do not pass all the way to that surface. In like 

 manner, if we inject from the uterine vessels, the placenta will be 

 rendered turgid, but nothing passes iuto the foetal vessels. From 

 this circumstance it is concluded that the placenta consists uniformly 

 of two portions : the one is furnished by the deciduous coat of the 

 uterus, the other by the vessels of the chorion, and these two portions 

 may, during the first three months, be separated from each other by 

 maceration. The structure of the foetal portion, so far as can be 

 made out, appears to be similar to that of the pulmonary vessels, the 

 artery terminating in the vein. But the maternal portion is somewhat 

 different ; there is not a direct communication, but the arteries, as 

 Mr. Hunter thought, seem to terminate iu irregular cells, and the 

 veins appear to commence with open mouths from these cells, for by 

 throwing wax in the uterine arteries we fill the cells, and frequently 

 inject the uterine veins also. 



It has always been considered doubtful whether the placental cells 

 of Hunter were real or artificial, being, in the latter case, produced by 

 extravasation of the injection ; and recent researches have confirmed 

 this doubt, but without throwing any satisfactory light on this very 

 obscure subject. With regard to the use of the placenta we may infer 

 that it is very similar iu man to what it is in ruminating and other 

 animals ; it most probably serves to produce a change in the blood of 

 the foetus analogous to that which the blood of the adult undergoes in 

 the Irogs : and, from considering that the foetus itself cannot create 

 materials for its own growth and support, we may further infer that 

 the placenta is the source of nutrition also. 



The navel-string, or umbilical cord, which connects the child to 

 the mother, is composed of the umbilical vein and two umbilical 

 arteries twisted together, and surrounded by a gelatinous substance 

 and the reflections of the chorion and amnios ; it also contains the 

 urachus, and the remains of the duct of the vesicula alba and ompbalo- 

 mesenteric vessels. It is visible in the human embryo in the sixth 

 week as a short and straight cord ; at birth the length of it is, on au 

 average, about two feet. The outer tunic of the cord, the amnios, is 

 continuous with the epidermis, or cuticle of the foetus at tho umbilicus ; 

 and in the same way the chorion, which is also reflected on the navel- 

 string, is continued into the dermis, or true skin of the foetus. 



The following is Valentin's account of the development of the prin- 

 cipal organs of the human embryo. 



" The primitive streak or groove is the first indication of the future 

 embryo. It consists of a very small longitudinal groove in the 

 middle of the upper surface of the serous lamina. It soon after 

 enlarges, while its two margins are raised to form the laminae dorsales. 

 They grow over towards each other, meet iu a longitudinal suture, 

 and thus inclose a cavity, the primitive tube. Anteriorly this tube 

 dilates into several vesicles, which lie behind each other, and in which 

 is deposited the cerebral substance. The spinal cord is laid down in 

 its remaining cylindrical poition. The several parts of the brain of 



