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REPRODUCTION IN PLANTS AND AXIMALS. 



REPRODUCTION IN PLANTS AND ANIMALS. 



the human embryo gradually pass through numerous transitional 

 forms, which corrwpond with their permanent conditions in various 

 at the lower animals. 



"A rlsnss cord, the chorda dorsalis, U early deposited beneath 

 almost the whole length of thin primitive tube. At the same time, 

 square poU are observed on either aide, symmetrically arranged in 

 pair*. Kach two corresponding squares subsequently grow towards 

 each other, to construct the body of a vertebra. In doing this, they 

 include between them a corresponding segment of the chorda dorsalis, 

 and gradually altogether displace it In Mammalia and birds, the 

 remaining portions of this structure subsequently disappear. 



" The vertebral arches commence as dense curved pairs of streaks ; 

 each of which unites on the one band with the body of the vertebra;, 

 and on the other with its opposite fellow. The various processes of 

 the vertebra) are only added subsequently. 



"The first rudiment of the skull is formed by a membranous 

 capsule, which gradually merges into a special cartilaginous covering, 

 called the primordial skull Some portions of the latter are ossified 

 immediately afterwards, while others disappear after new pieces of 

 bone have been opposed to them. 



The tlastema adjoining the inferior surface of the skull produces 

 scries of pairs of processes, which finally give rise to the chief 

 structures of the face and neck. Those which lie between the future 

 mouth and the chest are called the branchial or visceral processes; 

 and the fiuures which remain between them, the branchial fissures. 

 Their form and relations to the vascular trunks which supply them 

 somewhat resemble the type met with in the gills or branchial 

 respiratory organs of the fish. 



" The margins of the central portion of the serous lamina are 

 gradually involuted, so as to form the walls of the thoracic and 

 abdominal cavities. But as they only subsequently meet in the 

 inferior median line of the embryo (which wo are supposing to be 

 horizontal), there remains at first a long fissure, through which are 

 protruded the heart, a large portion of the intestinal canal, and the 

 allautois. This aperture afterwards closes in the region of the thorax, 

 and the posterior part of the abdomen ; and finally disappears, leaving 

 no relic save the navel The ribs commence as dense stria;, which 

 first become cartilaginous, and are then ossified. The several pieces 

 of the sternum are developed by a similar process. 



" The extremities are at first altogether absent They subsequently 

 sprout in the form of small stumps. Each of these is first divided 

 into an internal segment which pertains to the trunk, and corresponds 

 to the thigh or upper arm, and a free terminal plate which U developed 

 into the hand or foot The fore arm and leg are only formed sub- 

 sequently. The fingers and toes are at first united by a kind of web, 

 so as to resemble fins. This membrane begins to disappear from 

 without inwards. 



" The eye at first forms a hollow vesicle, which is connected with 

 the brain by a tubular handle, the future optic nerve. The retina is 

 produced from a deposit which resembles that of the cerebral sub- 

 stance in the vesicles of the brain. The crystalline lens, the vitreous 

 humour, and the iris, are only developed subsequently. A special 

 vascular tunic, the capsule-papillary sac, surrounds the lens of the 

 early embryo. Its anterior segment then forms the papillary 

 membrane, a vascular coat which U stretched immediately in front of 

 the papillary aperture. By the gradual loss of its blood-vessels, thin 

 ls conceited into a simple transparent membrane, which disappears a 

 fsw days after birth. 



" The labyrinth of the ear also begins as a hollow vesicle, having a 

 handle which is continuous with the brain. The vestibule, the 

 eochlfs., and the semicircular canals are then developed, at what is 

 comparatively a very early date. The formation of the auditory 

 ossidss is ultimately connected with the development of the most 

 anterior visceral arches. At this period the long process of the 

 malleus extends on the first maxillary process, or the future lower 

 jaw, as far as to the median line ; in the Mammalia, however, it after- 

 wards gradually disappears, so as to leave scarcely a trace. The tym- 

 panic cavity is chiefly developed from the gap situated at the first 

 visceral arch. The external ear is produced last of all. 



14 The organs of smell are also first indicated by vesicles which are 



connected with the brain. The nose U developed afterwards, during 

 the evolution of the face. The palate, which is subsequently laid 

 down, ends by separating the cavities of the nose and mouth. The 

 tongue grows out of the first maxillary arch. The external integu- 

 ment is only separated into corium and epidermis towards the end of 

 toe second month, or the beginning of the third. It afterwards 

 acquires Its nails, together with its various glands and hairs. In the 

 advanced nbryo, almost all the surface of the body is covered by a 

 very fin* down. The copious desqnamaUon and fatty secretion of 

 the >kin result in a caseous rabstanc* which coven many portions of 

 the festal body, and is capable of protecting it like an ointment from 

 the injurious action of the liquor aninii. 



" Those primary changes l.y which many of the embryonal organs 

 commence, are Acted without the aid of the vascular system. The 

 heart subseqaently begins as a tubs, which, anteriorly, is continuous 

 with centrifugal vessels or arteries ; posteriorly, with centri|>etal tabes 

 or veina. It afterwards undergoes a peculiar involution, divides into 

 segment*, produces the auricular appendages, and finally, presents two 



auricles and a single ventricle. The hitter gradually acquires a 

 septum, which is at first an incomplete, and finally a perfect one. 

 These embryonal vessels gradually undergo numerous changes, which 

 arc due, not only to the formation or metamorphosis of those organs 

 of the body that are rich in vessels, but also to a variety of causes 

 which belong to the vascular system itself. 



" The contrast of a systemic and a separative circulation obtains at a 

 very early date. A great part of the surface of the yolk u at first 

 covered by a vascular distribution, the aurea vasculosa, in which the 

 blood of the embryo U changed by a process, the details of which are 

 at present unknown. This vitellino circulation begins soon after the 

 heart of the embryo has commenced to beat In the Mammalia it 

 subsequently disappears, to make way for the placenta! circulation. 

 The blood then runs through the umbilical arteries into the foetal 

 placenta, where it undergoes a diffusion with the blood of the maternal 

 placenta, returning to the foetus through the umbilical vein. The 

 renovation thus produced corresponds, not only to the respiration of 

 the more developed being, but also to the most pressing requirements 

 of its nutrition. 



"The connection between the state of development of the heart and 

 that of the great vessels, produces a peculiar movement of the blood, 

 which has been designated the foetal circulation, or the circulation of 

 Sabaticr. It is most distinct shortly after the middle of pregnancy. 

 The blood of the right ventricle then passes chiefly into tli>' 

 half of the body and the placenta ; while that which returns from 

 this organ goes chiefly to the left heart, in order to flow thence to 

 the head and neck, from which it finally returns to gain the right 

 auricle and ventricle. So that there is a partial contrast between the 

 circulations of the upper and lower halves of the body. After birth 

 it is replaced by the systemic and pulmonic circulations. 



" The placenta! circulation ceases soon afterbirth, being replaced by 

 the pulmonic on the respiration of air. But in the normal course of 

 development the preparations necessary for this change are made some 

 months before the end of pregnancy. Hence, under favourable cir- 

 cumstances, a child which comes into the world seven or eight months 

 after conception may nevertheless continue to live. 



" The foramen ovale is due to the fact, that the inferior vena cava 

 originally opens into the left auricle, and not into the right, being 

 only gradually pushed over into the hitter. This explains why the 

 greater port of the blood that returns from the umbilical vein and tho 

 lower ports of the body passes into the left auricle during the fcctal 

 circulation. The groove which conducts it in this course is the relic 

 of a special adaptation, which dwindles and disappears in proportion 

 as the left auricle is claimed by the advancing development of the 

 pulmonary veins. Immediately after birth, the foramen ovale is at 

 first closed mechanically by the action of the auricle ; but it finally 

 becomes organically occluded. Tho superior and inferior vena cava 

 theu belong exclusively to the right auricle. 



" The pulmonary artery and aorta of the new-born infant are con- 

 nected with each other by means of the ductus arteriosus, or duct of 

 Botalli. This structure which is a necessary result of the develop- 

 ment of the embryonal vessels prevents the two divisions of the 

 foetal circulation being completely separated from each other, and also 

 hinders the perfect separation of the scarlet and dark-red blood in 

 the new-born infant whose lungs have begun to work. But in the 

 first few weeks after birth, the ductus arteriosus is closed by a process 

 which somewhat resembles that seen in a deligated artery. It is thus 

 converted into a ligameutous band, iu which form it remains during 

 the remainder of life. 



"After a certain period of embryonal life, the umbilical vein which 

 returns the renovated blood from the fcctal placenta, sends branches 

 to the liver. Besides this, it unites with tho portal vein, which also 

 ramifies in this gland. And it has also a certain communication with 

 the inferior vena cava, by means of a vessel the nervous duct of 

 Arantius which passes between the two. Hence, part of the purified 

 blood which is returning from the fcctal placenta can avoid the liver, 

 and flow immediately into the auricle. 



" The umbilical cord of the infant is usually tied and cut through 

 in some part of its course. The brute mammals gnaw it asunder as 

 soon as their young have breathed. After some time that portion 

 which remains attached to the belly dries up, and falls spontaneously 

 from the navel. Those portions of the unbilical arteries which first 

 run along the bladder, and theu ascend on the abdominal walls to the 

 umbilicus, become converted into ligamentoua tissue. The duct of 

 Arantius and a large part of the umbilical vein also experience tho 

 same fate. 



" The development of the intestinal canal commences by the centre 

 of the mucous lamina being raised and folded inwards. In this way 

 it produces an intestinal groove, which is open towards the yolk. 

 Thin groove hi soon afterwards shut off anteriorly and posteriorly, so 

 that there only remains a median gap, tho intestinal navel The cir- 

 cumference of the mucous lamina furnishes a covering for tho umbili- 

 cal vesicle. The portion which intervenes between this and tho 

 intestinal navel is drawn out into a cylindrical duct, the pedicle or 

 stalk of the umbilical vesicle." (Valentin, ' Text- Book of Physiology.') 



The fcctus has many peculiarities which distinguish it from tho 

 child after birth, most of which are peculiar to its mode of life, and 

 are lest immediately after being separated from tho mother, or are 



