1897 Human Embryology 3

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Minot CS. Human Embryology. (1897) London: The Macmillan Company.

Human Embryology: Introduction | The Uterus | General Outline of Human Development | The Genital Products | History of the Genoblasts and the Theory of Sex | The Germ-Layers | Segmentation | Primitive Streak | Mesoderm and the Coelom | Germ-Layers General Remarks | The Embryo | The Medullary Groove, Notochord and Neurenteric Canals | Coelom Divisions; Mesenchyma Origin | Blood, Blood-Vessels and Heart Origin | Urogenital System Origin | The Archenteron and the Gill Clefts | Germinal Area, the Embryo and its Appendages | The Foetal Appendages | Chorion | Amnion and Proamnion | The Yolk Sack, Allantois and Umbilical Cord | Placenta | The Foetus | Growth and External Development Embryo and Foetus | Mesenchymal Tissues | Skeleton and Limbs | Muscular System | Splanchnocoele and Diaphragm | Urogenital System | Transformations of the Heart and Blood-Vessels | The Epidermal System | Mouth Cavity and Face | The Nervous System | Sense Organs | Entodermal Canal | Figures | References | Embryology History

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The Genital Products.

Chapter III. The History of the Genoblasts and the Theory of Sex

The term genoblast is used to designate the sexual elements. I apply it exclusively to sexual elements proper, and not to the accessorj' parts with which those elements are associated. The spermatozoon is a genoblast; a spermatophore is not. The egg-cell after maturation is a genoblast, but not before.

I. Spermatozoa

1. Summary. — The spermatozoa of mammals are filaments consisting of a short, thick end called the head, and a very long and delicate thread called the tail. The head varies greatly in shape, according to the siHJcies; in man it is broad and thin. Fig. 22, and is widest at a little distance from the tail. The head contains chromatin, and may be colored by the usual nuclear dyes. The tail consists of three parts: 1, the middle-piece., which is next the head, and the thickest of the three parts; it contains an axial thread, and probably always has a vern lino spiral thread running round it; 2, the main-piece; and, ^3, the end-piece ., which is not more than a line, even as seen with xery high magnifying powers. The human spermatozoon is 0.055 mm. long — the head being 0.005 mm., the tail (».050, and the middle-piece 0.000.

The development of the mammalian spermatozoa begins with a socalleil parent or mother-cell, which lies near the outer wall of the seminiferous tubule. The mother-cell produces a number of daughter-ceils, which also multiply by division ; the daughter-cells brealc down, forming a column of matter (protoplasm), in which lie their nuclei, and at the base of which lies the nucleus of the mother-cell ; the nucleus of the mother-cell and the column of matter both ultimately disappear, but exactly how is not determined ; the nuclei of the daughter-cells produce each a spermatozoon. The head and tail of the future si)erniatozoon become visible within the nuclear membrane ; the head is formed chiefly by the chromatin of the nucleus ; the nuclejir membrane finally ruptures, and it as well as the contents of tJie nucleus which have not taken part in the formation of the s|)ermatoz<x)n are lost. Among the lost parts is a special round body of small size, which appears in the nucleus while the si)ermatozoon is developing ; this body may he stained by chloride of gold, but not by ha^matoxylin ; its significance is unlmown. The long colunm holding the spermatozoa together has usually been regarded as a c^ll, and is the supporting cell auct., or Sertoli's column.

2. Spermatozoa are the essential fertilizing elements secreted by the male ghmd. They are minute Ixxiies, capable of active locomotion, and having a characteristic form in each species. In a few instances (certain snails, etc.) there are two distinct forms of spermatozoon for a single species, but usually there is only one form, and that little variable. In a small number of animals the spermatozoa, as in the nematods, are distinctly cell-like ; but in the great majority of animals, and, so far as I know, in all vertebrates, the}' are long and thread-like ; hence their common German name, Samenfddeii, first proposed, I think, by Kolliker.

The mammalian spermatozoa are long, slender bodies, varying considerably in configuration, but all presenting at least the following features in conunon : One end is thickened and is called the head ; it has a strong affinity for nuclear staining fluids ; this affinity must be attributed to the chromatin, which the head contains, as is sliown by the history of its development ; the remainder of the spermatozoon is long and slender, and constitutes the tail ; the tail consists of — 1, a middle part (Mittelstuck)^ a little thicker than the rest, and situated next to the head ; the middle part is traversed by a verj" fine axial thread, and ends abruptly; and, 2, a hind-piece, which, according to some writers, may be subdivided naturally into two segments, the main-piece (Hmtptstnck) and end-piece.

The spermatozoa of the various species differ in size in the proportions of the parts, and often very strikingly in the shape and structure of the head ; those of the opossiun are esi)ecially remarkable for being double ; two apparently complete spermatozoa being united to a common plate by their heads (Selenka : ** Studien liber Entwickelungsgeschichte," Heft IV., p. lOG). Twin spermatozoa have also been observed in the rat by Neumann, 75.1, 313, Taf. XVII., Fig. 10, b. Compare also Max von Brunn, 84. 1, and Brock, 87.5.

The largest known mammalian spermatozoon is perhaps that of the marsupial, Phascogale; the spermatozoon of this animal is 0.263 mm. long — the head, however, being only 0.013 mm. (Fiirst, 87.1, 354). The spermatozoon of the rat is 0.144 mm. long, the head 0.009, the tail 0.135, and the middle-piece 0.045 mm.

La Vallette, 71.1, gives a synopsis concerning the forms of vertebrate spermatozoa nearly as follows : Fish : The spermatozoa of Amphioxus are threads with round heads. In Petromyzon the head is rod-like or egg-shajjed. The teleosts generally have pin-like spermatozoa ; but in the salmonidae (Owsjannikow) the head is pointed and shaped like a heart-tip. The spermatozoa of selachians are much larger, with the head-end spindle-shaped and often spirally twisted. Amphibia : The head is long, generally pointed, the middle-piece short, and the tail is often provided with an undulatorj' membrane (Retzius, 81.1). Reptiles and birds : The head is usually long, often twisted. Mammals : The head is more or less elongated ; in ungulates the head is flattened and usually more or less egg-shaped in outline, the pointed end toward the tail. Among rodents there is considerable variety of form. In the dog the head is pear-shaped ; in the hedgehog the head is truncated inferiorly , and the tail is inserted laterall}'. No comprehensive summary of the observed forms of spermatozoa has been made since the publication of Wagner and Leuckart's article in "Todd's Cj'clopaedia.'

The most minutely studied mammalian spermatozoon is that of the rat, thanks especially to the patience of O. S. Jensen, whose posthumous paper, 87.1, furnishes the basis of the ensuincf d'^scription. The rat's spermatozoon measures 144 /i; its head. Fig. 21, C, is a broad hook, pointed at one end and obliquely truncated at the other ; from one corner of the truncated end starts the very long slender tail, which is divisible into the thicker middle-piece (Mittelstiick, or Jensen's Verbindangsstiick) and the thinner main-piece (Hauptstilck)^ Fig. 21, A, which terminates in a short and still finer thread called the end -piece (Endstilck) . The appearance of the spermatozoon varies according to its degree of development, it not attaining full maturity until it has left the seminiferous tubule. The changes referred to jiffect principally the head and the middlepiece. The head is covered, while the spermatozoon remains in the seminiferous tubules, by a membranous cap. Fig. 21, A, which subsequently disappears. The middle-piece has a spiral thread running round its outside. Fig. 21, B. The spiral thread appeal's soon after the rupture of the nuclear membrane, by which the developing sj^ermatozoon is set free (cf. infra). The thread is at first indistinct and makes only a few turns ; it rapidly becomes more distinct and the number of turns increases, until they become so numerous that in a spermatozoon taken from the vas deferens onh' a series of thick-set cross-lines can be distinguished ; these lines have been seen by several observers and variously interpreted ; the spiral may run to the right or to the left. The thrend becomes loosened off by the action of glycerin (1 part) and water (4 parts) , and is destroyed in one to two hours by 0.0 per cent salt solution, leaving then the axis imcovered. The thread can be stained bj^ chloride of gold, though the axis cannot. The axis, when the spermatozoa are treated with acetic acid, often breaks up into threads {cf. Ballowitz, 86.1); it shows a lighter line in its centi-e. These observations lead Jensen to the conclusion that the axis is formed by a wall of fi brilte surrounding a central core or cavity. The axis does not reach cjuite to the head, but ends with a little knob, leaving a small, perfectly transparent space between the knob and the head. Fig. 21, C. In some spermatozoa — e. g., of horse and ox — though not in those of the rat, there is a minute opening in the head called the microporus, and situated just opposite the knob of the axis. When the spermatozoa are stained with nuclear dyes, most of the head is. colored, but the tip of the hook, which contains no chromatin, and is probably formed out of a scrap of the nuclear membrane, remains uncolored : on the concave side of the tip a fine line can be distinguished, due, apparently, to a rod of substance. Sometimes a minute fragment of the nuclear membrane is left adherent to the lower end of the middle-piece; for the explanation of this possibility, compare the section below on development.

Fig. 21. — Structure of a rat's spermatozoan. B. young spermatozoon, end of the middle-piece and Ix?ginning of the mainpiece to show the spiral thread— great Iv magnified ; A, head, with part of the axial thread ; C, immature spermatozoon, a n t erior half only. After O. S. Jensen.

The human spermatozoa are described by Retzius, 81.1, 85, as follows: The head, seen from the flat side, appears oval. Fig. 22, A, with the front end generally tapering a little, but never pointed; the anterior half or two-thirds has a brighter and more transparent part. Seen on edge, Fig. 22, B, the head has a pointed form, with a posterior thicker, round, dark part. By adjustment of the focus it can be ascertained that the sides near the point are depressed somewhat like those of red blood corpuscles. Retzius could nowise succeed in demonstrating a special tip (Spiess) corresponding to that in the salamander, but Edw. M. Nelson {Jorirn, Quekett Chib^ 1889, III., 310) has observed a slender thread prolonged from the head, and also a hook at the end of the thread ; these observations have been confirmed by Bardeleben, 91.1. The latter also describes additional details of the structure of the head. The following piece (Schweigger-Seidel's Mittelstuck) is directly united with the head by a transverse joint; there is no neck in Elmer's sense; the middle-piece is cylindrical and relatively small — about as long, or a little longer, than the head; its surface is often granular or rough, and there cling to it a few shreds of protoplasm, as has been described by several investigators ; the spiral thread was long overlooked, but has been recognized and figured by K. Bardeleben, 91.1. The undulatory membrane, supposed by Gibbes, 79.1, and W. Krause, 81.4, to be present, was perhaps an abnormally loosened spiral thread. The main-piece of the tail is about half as thick as the '* Mittelstuck," gi-adually tapers, and ends abruptly at the beginning of the still finer and very short end-piece,

8. Spermatogenesis. — The seminiferous tubules are cylindrical, i. e., in cross-sections they appetir round; a large part of the tubule is filled with spermatozoa in various stages of development. The outer boundary is marked by a distinct line corresponding to the tunica

?ropria, a layer of endothelial cells, with flat oval nuclei (Neumann, '5.1, 306) . Next to the tunica comes a layer which, as far as known, presents pretty much the same appearances, whatever may be the stage of development of the spermatozoa within. This layer contains two kinds of cells: i^/rsf, the large Sertoli's columns, as they may be called, after their discoverer.* These cells are identical with Merkel's Stutzzellen, La Vallette's spermatogonien, Swaen and Masquelin*s cellules foUiculaires. Second, small granular cells, varying in appearance according to tho exact st^e of their development. Examined in surface views. Fig. 23 (compare also Figs. 5, (i, and 41 of Furst's paper, 87.1), the large cells are seen to be mostly hexagonal in outline, to touch one another, and to pass below, /. e. outwide, the small cells; they have large, clear, oval nuclei with sharp outlines, and usually a single well-marked nucleolus. The nuclei lie quite near the tunica propria, but in man lie farther inward, aud are in this case not so near the tunica as are the smidl cells. Around the uucleiia there lie a few highly refractile granules which may be stained by arsenic acid, and are probably fat. The small cells lie in depressions or cups of the largo cells. Fig. 33, B, and when the small cells are knocked out — as sometimes happens in teasing — the partitions between the cups appear more distinctly and create a network tigure, which formerly misled Vun Ebner and others into describing a real network as constituting the layer. The large cells also have long columnar prolongations, as can be best seen in transverse sections of the tubules. Fig, 2; the prolongations are united with bundles of developing spermatoblasts. The svtttll cells are very different; they lie over the outlines of the large cells and between their centripetal prolongations. Fig. 211; tbey are granular, have comparatively little protoplasm, and their nuclei are nearly spherical in shape. The nuclei vary considerably in appearance, as these cells multiply by indirect division ; usuaUy they contain a chromatin network or a coiled chromatin cord; sometimes the network is concentrated at one side of the nucleus leaving the other side comparat el clear 4t certa n per ods the nude are found in varioiis stages of kar^ ok nes s The cells result ng from the division of tho small cell f m tl o packing between the nward columns of the lar^o cells lee cr ect ns e get alternating Columns, Fig. i

Fig. 22.— Human spermatozoa. A, complete spermatozoon ; B, head seen from the side ; C, extremity of the tail. All highly magnified. After Retzius.

  • First describe<n»v Sertoli in ii. Morijagni (rf. Henle'sJahresberichte for 1864, p. 190). Compare Sertoli, Arch. Sci. mediche. ii., IW (1877^.
Fig. i

The descendants of the small cells prduce the sperm blunts, and the spermatoblasts axe converted into the si>er natozoa. The smdll ^J^ -^ -^oJii*^S^ cells are then tl o \j) parents of the sjie matozoa and maj be

called the parent .

CfUs; a great variety of names have been employed to designate the ucl a mother cells spore cell ger minative cells, Saonenstammzellen etc The nomen latu e of tl e small cells is very confused; those of them m process ot mdirect division are often smaller than tho others and have been designated us the growing cells" by H. H. Brown, 86.1, and this term has been used by other writers since. The small cells in the resting stage are called " Stammzellen " by most German writers, as an equivalent for which I have adopted pareni-cell.

Formation of the Spermatoblasts

The parent-cells divide and produce probably tbree cells, although the number has never been accurately ascertained. One cell reraaina as a parent-cell, and the other two are the mother-cells {Mutterzellen) and are well characterized by their appearance. According to Biondi, 86.1, the nucleus of the parent-cell remains and becomes like the nucleus of the lat^ cells (Sertoli's or supporting cells). The mother-cells divide and their descendants also divide until there is produced a column of cells, Fig. 24, which stretches in a radial lino from the mothercell toward the centre of the tubule, and is packed in between tlio columnar centripetal prolongations of Sertoli's cells {cf. Figs. 24 and 29). Probably, then, although investigators are not agreed in regard to this point, the parent-cells divide in such a way that the cells resulting from the division aie unlike, one of them preserving the character of the parent-cell, and the others differing from it in ha\ing a relatively larger nucleus and a finer chromatin network; the appearance of the nuclei varies, of course, according as they are in the resting or divisional (kinetic) phase.*

The cell most like the original one, and which we may call still the parent-cell, lies at the outer edge of the tubule, while the others or mother-cells lie toward the centie. Fig. 24. The parent-cell, as already stated, produces at least a second and perhaps more mother-cells, so that the column grows centripetally. The column also grows by multiplication of the mother-cells, but the cells thus formed lie in the innermost part of the column; they are smaller. Fig. 24, than the first generation of (mother) cells; they have relatively large nuclei, with the chromatin gathered into two or three spots — nucleoli. We thus have a column of cells in which we can distinguish three zones: 1, the outer zone of the parent-cell; 2, the middle zone of the mother-cells ; 3, the inner zone of the daughter-cells. These zones remain more or less markeil for a considerable period ; for, as the cells of the inner zone change int^i spermatoblasts, those of the middle zone change into second daughter-cells, and as the inner spermatoblasts Siiiodir'x uoi'dmnis' change into spermatozoa the cells of the second zone change into spermatoblasts; the innermost zone long continues one stage ahead. The trizonal arrangement is verj' conspicuous in cross-sections.

The division of the mother and daughter-cells pi-esents many peculiarities, and does not conform exactly to Flemming's well-known scheme of phases for indirect division. Attention was first directed to these peculiarities byCamoy, in an important memoir, 85.1, and "W. Flemming, 87.1, has since confirmed these discoveries, in lai^ part, by observations on the salamander, and gives a plate of diagrams which is instructive as a facile means of comparison. La Valletta, Niessing, 88. 1, p. 44, and others find that when the mothercells multiply there is often a stage to be found where several nuclei (two to twelve) lie within one large cell. The multinucleate giantcells are best found by teasing the fresh specimen. As to their place in the spermatogenetic history we possess no definite knowledge.

The spermatoblasts arise from the nuclei of the daughter-cells (spermatocytes), and not as H. H. Brown, 85.1, and many others have, I think, erroneouslj' believed, each out of a whole cell. Biondi, 85.1, seems to me right in his statement that the bodies of the cells break down, or at any rate lose their boundaries, thus creating a granular protoplasmatic column in which the nuclei lie. Compare also Niessing, 88.1. The protoplasm of the parent-cell participates in these changes, hence its nucleus comes to lie at the base of the colmnn. This nucleus has meanwhile altered its character, and become large, clear, and nucleolated. Now, these columns are the same as the large Sertoli's or supporting cells above described. By no means all writers agree with this account of the origin of Sertoli's cells, but all other explanations that I have found appear to me vague and confused, and the history of the changes here advocated is clear, and accounts for the well-established grouping of the spermatoblasts in the substance of Sertoli's column ; this essential phase is explained satisfactorily by no other theory.

The nuclei congregate at the inner end of the column, and there change their character and fi«. as. -beveiopiuK «ix'riiiatobitt8t« of the rat :

become recognizable spermatoblasts, Figs. 25 and 29.

Development of the Spermatoblasts into Spermatozoa

The nuclei change into spermatozoa as follows: The chromatin is at first unequally distributed throughout the nucleus; it then in great part accumulates at the end of the nucleus toward the outer wall of the tubule ; at this stage the chromatin is densest near the equator of the nucleus, where the edge of the chromatin is sharply marked, and toward the outer pole of the nucleus the chromatin is less condensed (Niessing, 88.1, p. 40, Taf. I., Figs. G, 7, and 8). It is from the equatorial plate that the future tail grows out at the start. Particles of the chromatin are said to remain in other regions of the nucleus, and finally to gather together to form the small accessory corpuscle mentioned below. According to Platner, 89.2, 131, 132, the portion of the nucleus which forms the head of the spermatozoon in pulmonate snails is homologous with his Nebenkern, The main mass of the chromatin is concerned in the formation of the head of the spermatozoon ; it is at first quite round, Fig. 25, a and 6, but soon begins to .'liter its shape, gradually assuming the form of the si)ermatozoon head. Fig. 25, c, c/, e, /. The tail appears very early as a delicate filament, growing out from the chromatin and lying entirely within the nucleus, Fig. 25, a^ but shortly after is found to project beyond the nuclear membrane, 6, and lengthens rapidly, e, /, g. The nuclear membrane is very distinct ; it elongates into an oval bag, 6, c, one end of which lies close against the chromatin, while the other surrounds part of the tail and is wide; the lengthening continues, ^5 ft Qy with accompanying changes of form, best indicated by the figures ; the part of the tail within the nuclear membrane becomes the middle-piece, Fig. 2r>, but the spiral thread is not developed until later. The accessory body may be readily seen in the rat ; unlike the chromatin of the head it can be stained by chloride of gold : hence, if it is formed of chromatin at all, the chromatin must have undergone alteration. Finally, the nuclear membrane ruptures, Fig. 27, a portion of the membrane remains upon the head, and the caudal bag sometimes endures longer. Fig. 25, r/, but at last also disappears, except that in certain cases a trace of it remains visible as a fine cross-line at the end of the middlepiece.

Fig. A>.— Devi'loiiiiiifsiH spermatozoa of .a niursu]>ial: Mtttichirns (^lica. A, B, C. different stages. After Ftirst.

Fiirst and others think that the axis of the tail is formed from the chromatin, and that the sheath of the axis arises from the achromatine substance of the nucleus (carj^oplasma) .

After the rupture of the nuclear membrane the young spermatozoa still develop a little farther. The spermatozoa are ulti mate lylil)erated, and, falling into the lumen of the tubule, pass off.

From their mode of development, it is evident that the spermatozoa necessfirily lie in bimdles, each bundle l^ing held together by a Sertoli's column, Fig. 28; at first they lie at the inner end of the column, at a considerable distance from the basal nuelexis, but as the nuclei (sjiermatoblasts) lengthen, the heads push their way toward the base of the column. Fig. 'Z'.i. Now as the development of the daughter-cells (spermatocytes) is continually progressing between Sertoli's columns, we obtain in sections the long-known, remarkable appearances shown in Fig. 3!i, of bundles of spermatozoa alternating with coluuuis of proliferating cells.

Fig. 27. - lluiiuiti Kpri'Mmtot)lastH, lo illustrat»'the rupture of the membrane. After Wittlersijerj;:.

4. Historical. — The seminal animalcules were, it is state<l, first discovered by Ludwig Kumm, then a student at Leyden, in August, IfiTT. Leeuweohoek claimed the merit of having made the discover^' in November of the same year, and in I0~8 Hartsoeker published an account of them, professing to have seen them as early as 107-1. They wei-e long considered to be prolmbly parasites, and it was not until Prevost and Dumas' researches that it was definitely ascertained that the " animalcules " were the essential fertilizing element. Thus Richard Owen, in his article on "Entozoa" (If^'ifi) in Todd's " Cvclopfedia " include"* the spermatozoa n lor tl at head, Itho gh he writer It i

still u let 1 i tlertl

are to be re^ardela. ai 1 gous to the no ng tilameuts of the jwllen of plants or as nde

toiilBEts, After

dendent rgan sms (\ II., p. 41 ) But just after 1 o dd Al tbougl n 1 itin t oi^.ins of gei e

tion have been detected there reason to suspect that the spelozoa are oviparous; th re also stated to i>ropagat spontaneous fission, the s aration taking place between the disc of the body and the caudal appendage, each of which develop the part required to fonii a perfect wTiole."

meanwhile the investigations of Spallanzani, Wagner. Czermak, and many others gradually increased the knowledge of the forms of the spermatozoa. Dujardin was the first to consider the spermatozoa as generated from the inner layer of the seminiferous tubules, and therefore not as parasites. The discovery of the si)ermatoblasts or immature spermatozoa by Von Siebold (Miiller's Archiv, 1836 and 1843), soon confirmed by Koliiker and Reichert, marks an important step. Now follows a series of publications by which one detail after another was added to our knowledge. During the past twenty years there has been rapid progress, which may be said to have begim ^vith Schweigger-Seidel's important memoir, 65.1, and to have made us acquainted with the minute structure of the spermatozoa, and their development. Another line of investigation was opened by O. Herwig,(1875), in following up the history of the spermatozoon within the ovmn after imnregnation. For further historical data, see Waldeyer's address,

II. Ova

Definition. — The term or urn is employed in various senses. It is applied — 1, to the cell distinguished as the ovarian cell, or immature ovum, out of which the female product or mature ovum is developed ; 2, to the mature ovum, or true female spore; 3, to the mature ovum plus the fecundating spermatozoon united with it — that is, to the impregnated ovum; 4, to various stages of development of the embryo. In this article we consider only the ovum in the strict sense — namely, as the female sexual product.

Summary. — The ovum arises as a cell, which matures by a series of changes, of which the last and most striking is the expulsion of the so-called polar globules; there are many important changes which occur earlier. The genesis of the mature ovum maj'^ be conveniently divided into three arbitrar}' stages : (1) Differentiation of the ovic cell; (2) growth of the cell and accumulation of nutritive material in it; (3) maturation proper.

1st. The Origin of the Primitive Ova (Ureier or Ovic Cells).* — It seems to me, in the light of the recent investigations of the origin of the ova in vertebrates, safe to assert that they arise from cells of the mesothelium (j)eritoneal epithelium) covering the genital ridge of the embryo, the ridge giving rise to the adult ovary. On account of its function the epithelium of the genital ridge has been called the germinal epithelium (KeimepitheT). In mammals, which alone will be here considered, according to the best authorities the ureier are developed as follows : Certain cells of the germinal epitheliimi become larger than the others ; these cells are soon carried into the interior of the ovary by being included in cord-like ingrowths of the epithelium. These cords are the PflUger'schen Schlduche of German writers. The primitive ova exist in multiple in the cords, but each of them early l)e(*()ines surrounded by a separate envelope of epithelial cells. A little later each ovum separates from its neighbors and appears as a round cell, with a clear nucleus and distinct nucleolus, /, closely investeil by a layer of cells smaller than itself. The yoimg egg-cell, together with its epithelial envelope, constitutes the so-called primordial follicle (Fig. 31).

  • For a full discussiuD of tliib subject see Clmpter XXIII.

2(1. General Growth of the Ovum and Development of the Yolk. — The modifications which occur in growing egg-cells are as follows : 1st. Change of size; the cell enlarges^ it being a rule — no exception to which is, I believe, kno\\Ti — that the mature egg-cell is much larger than any of the other cells in the boiiy of the parent. 2d. Change of shape; the cell usually becomes nearly or quite spherical; the shape of the egg does not necessarily remain spherical, but may l)e altered b}' external pressure, as in the uterus of Arion ("Hdbk.," Vol. IV., p. 7, Fig. 1H15), or as when several are laid in one ca-i^sule (Lumbricus, Nephelis, Planaria, etc.), or w^hen compressed by an unyielding shell. An instance of the last-mentioned kind has l^een described by Repiachoff (Z.f.triss. Zool., XXX., Suppl.), who figures the egg of a European bryozoon found on eelgrass as fusiform. Fig. 30. 3d. The nucleus becomes larger, spherical, and aasmnes an eccentric iX)sition ^vithin the cell; the chromatin usually gathers into one nucleolus, as in mammalia ; the nucleolus is large, distinct, highly refringent, easily stained, and placed eccentrically within the nucleus. The achromatic substance or protoplasm of the nucleus develops into a coarse network, which radiates irregularly from the nucleolus as a centre. 4th. The cellular network becomes very distinct; its interspaces l)ecome filled with ovoid, round or crystalline solid inclosures, which are usually, if not always, mainly of an albuminoid character. The inclosures form the i)art which is called the deutoplasm by Edouard van Beneden and others, Fig. 3o.- e^^ of Tendra The deutoplasm is the same as the yolk- substance Xff^'^^Ma^ifl^ ^^^^' of older writers, and is a store of nutritive material from which the protoplasm draws subsequently to support its growth. The term yolk has no very exact meaning, for it is used to designate sometimes the deutoplasm alone, sometimes the whole ovum pro|)er, as when the segmentation of the yolk is spoken of. 5th. In all vertebrates an ovarian envelope, the zona radiatay is forme<l. 0th. It is probable that a vitelline or true cell-membrane is always formed inside the zona by the egg-cell before it reaches maturity.

Primordial Ovum.* — In the ovarj- at birth, and thereafter up to the period of the climacteric, are small egg-cells, some of which develop from time to time into mature ova. At all ages these small egg-cells together with their follicles present a constant appearance. It is currently stated in text-books that there are some seventy thousand egg-cells in the human ovary at birth ; but upon what authority this assertion rests I do not know. In any case the number is very large, and it is probable that a good many of them never develop, but degenerate. These youngest egg-cells are known as the primordial ova; they lie in a layer immediately below the alhugiuea of the ovary and never in the medullary region. Fig. 32. They are slightly irregular globules, usually 50-On n in diameter; 48 by 54 /i, 54 by 5S /i, 04 by 08 /i, exemplify actual measurements. The protoplasm is finely and evenly granular, and consists of a uniformly clear matrix (hyaloplasma) and a fine reticulum, which may be brought out by eosine staining. In birds His, 68.1, has found protagon " granules in the primonlial ovum, and Ed. van Beneden affinns, 70.1, that yolk-grains are present in the primordial ovum of various mammals ; but in man this is not the case. The protoplasm is naked^that is, not enclosed in a cell membrane. The nucleus is round, lies in the centre of the cell, measures from 29 to 32 /* in diameter, is lx)unded by a very distinct membrane, and contains a roimd excentrically-placed nucleolus, about 9 i>- in diameter. Between the nucleolus and the membrane there is a loose network of fibres, attached to both; the substance of the network is different from that of the nucleolus, as is shown by its different staining. The network was first observed by Flemming, 75.1, in Unio and Anodonta, and has since been often observed in many species ; it was first described in human ova by Trinchese (Mem. Acad. Sci. Bologna, Ser. III., T. VII.). Some of the primordial ova of veiy young children have no nucleolus, and in bats all of them are at first without it according to E. van Beneden. The position of the nucleolus is variable; it may lie cl<:)se to the membrane of the nucleus or nearly in the centre. A peculiarity worthy of mention is that once in a great while a primordial ovum has two or even three nuclei. This occurs so verv rarelv that it cannot be considered as any evidence of multiplicati(^n of the ova, but only as an extremely abnormal variation (see Nagel, 88.1, 372-3T5). Each primordial ovum is surrounded by a very thin epithelial envelope, t ig. 31, /, with scattered fusiform nuclei easily distinguished in stained specimens from the similarly shaped nuclei of the neighboring connective tissue.

  • In the ensuing aocoiiut of the ovariau ovum up to its maturation, I have been uruided chiefly by Nagels article «8 1

The shape of the follicular nuclei has misled Schron, 63. 1, Foulis, 76.1, Klebs, 63.1, and others into maintaining that the follicle is derived from the stroma-cells, instead of, as is really the case, from the germinal epithelium ; KoUiker traces the origin of the follicular cells to the "' Ma rkst range ^^ ; others, as, notably, Harz, 83.1, and Sabatier, 84.2, derive the follicular cells from the ovum. Both views must, it seems to me, be discarded (compare for details, Cliapter XXIII.). The follicle forms a closed wall around the entire ovum, and not one with an opening, as certain authors have maintained. The primary' follicles of mammals were first described by Barr>% 38.1, under the name of ovisacs; his observations were soon confirmed by Bischoff, 42.3. They are now familiarly known to all histologists.

Growth of the Ovum and Primary Follicle. — The follicles remain for a long time without change, but from time to time certain ones of them develop. In a mature ovary we can find always several stages. The cause of the development of the follicles is unknown. The primary follicles are always near the surface; as they grow in size they move deeper into the stroma. The first step is the multiplication of the cells of the follicle. Fig. 31, A, which converts the follicle into a layer of cubical cells with the nuclei at an even height. During this change in the follicle the primordial ovum does not alter in size.

The second step is the elongation of the cells into a cubical form, with an accompanying enlargement of the ovum. The growth of the ovum tifftH.:ls the pivtoplaaiii, thti iiuolens, and the imoleohis, all of which increase their di mens ions. The follicular wall steadily increases in thickness; at first it remains single-layered, but the nuclei take their places at various levels; a little later it becomes several-layered, and then the formation of the first envelope {zona pefhicida) uniund the ovum liegiiis, Nagel, 88.1. 38(1-382, calls attention fc» the laryo clear cells with lai^ nuclei, which show a distinct reticulum and one or several chromatin gianiiles; they are found in somewhat larger follicles, but only up to the time when the yolk granules begin to form in the ovum. Xagel int^'rprets these cells as having a nutritive function, and calls them ynhrzplfeii: he offers verj- little evidence in favor of his view. The (relU in <niestion measure 10^l It, and are much smaller than the primordial ova, which they sttmewhat resemble pia.s nmryfn egfron uw in appearance. These cells have been °fJP' l^" ^^ ^ "mS"" i^w* seen by various authorn, c.j/., Call and /.<•],»} a fo e --((nniiiit Exner (Sitzljer. Wien. Akad, Wiss., m"nV/I^e vmicie, Aflw'ft^Niw^. etc., IT} April, 1S(J,5). It is more probable that these cells have to do with the formation of the liquor of the Graafian follicle, whicli begins while they are present. The cells of the granulosa midtiply by indirect division, as has been shown by Harz and alwo Flemminy (Arvh. mikrusk. Anaf., XXIV. 37(j-3S4). I have found the numerous kar>'okinetic figures in the follicles of the rabbit's ovary, though hai-dly quite as abundant as Flemming's description led me to expect. The mitoses have not been found in the first stages of follicular growth. During the growth of the follicle there is formed, as was finst described by Schriin, 63.1, ii'.\ a network of bl<Kxl-ve.'?sels close around the follicle; the laver of blood-vessels constitutes the so-cjdled tiitiica i-asctiIfiftn or thera foUicnli : the first vessel is a simple lixtp, which embraces the young follicle; other loops approach and unite with their fellows to form a network.

Development of the Q-raaflan Follicle. — After the epithelium of the primarj' follicle has become many -layered, there appear in it rttunded vacuohited spaces, which increase in size and finally become confiuent, so that there is a space or fissure in the epithelium, Fig. 3'^, 4- This fiasnre divides the epithelium into two layers, an inner one immediately surrounding the ovum, and an outer one next the stroma of the ovarj*. Since the fissure does not extend completely arotmd the follicle there is one place where the two layers are united, Fig. '-Vl; the place of union, though variable in position, is always on the side of the follicle awa>' from the surface. The fissure is fiUetl witli a serous fluid known as the liquor foUinili. In man and most mammals there is a single continuons fissure; hut in the rabbit, and perhaps other rodents, thei-e Jire often cords of cells stretching across from the outer to the inner lamina of the epitheliiun ; the cords vary in number from two to ten ; they were first ilescribed by Barrj', have been beautifully figured by Coste, 47.1, Lapin, L. I., Fig. 2, and are known as the ret inacula. The development of the fissure changes the primary into a Graafian follicle.

The Graafian follicle is bounded by a layer of epithelium known as the meinbrana grmntJnsa, from its appearance when examined in the fresh state ; it is surrounded by a vascular layer, characterized not only by its blood-vessels, but also by the condensation of the connective tissue composing it. The follicle lies a little below the layer of primordial ova. To a part of its walls on the side away from the surface of the ovary is attached a mass of cells more or less globular in shape; this mass is known as the (Uncus or cumulus projigervs; it encloies the ovum ; the cavity between the discus and granulosa is the cavity of the follicle, and contains the liquor folliculi. The further history consists principallj- in growth and secondary- modifications. The follicular wall and the discus increase in thickness*, there is added a very thin basement membrane, Waldeyer's membrana propria, close arotiiid the outside of the granulosa and separating it from the tunica vasciilosa; the membrana pn>pria is said to be an endothelium derived from the connective-tissue cells of the ovarj'. The vascular membrane or theca folliculi becomes differentiate<l into an outer fibrous layer (Henle's i'liiira Jibromf) carrying the larger blood-vessels, and an inner less fibrous layer carrj'ing the smaller blood-vessels {tiinlcn jiroprin). Tlii' distinction l»etween the membrana pmpria and tunica propria should not be overlooked. The smallest bhxxl-vc'ssels running around the follicle from below, and minutely suMivideii on its upi)er surface, converge toward a {>oint near the sxirface of the ovary: this \mm\. is called the stigma, contains no blood-vessels, aud marku the siw)t where finally the follicle is to nipture to allow the ovnm to escape, the stigma, owing to the absence of blood-vessels, is yellowish-white. In mammals and birds it is elongated and rounded in outline, but in lizards is angular (Coste, 47. 1, 100) . The cells of the granulosa acquire, at least in the cow, highly characteristic forms (Lachi, 84.1); there are, 1st, very narrow elongated cells, which stretch through the entire thickness of the layer, and present, when isolated, curious irregular forms; they have oval nuclei, about which there is usually a small amount of protoplasm ; the nuclei of these cells lie in the half of the granulosa next the cavit}^ of the follicle. 2d, cells with rounded nuclei, larger cell-bodies, and a few fine processes of irregular shapes ; these cells lie between the processes of the others in the outer half of the membrane. 3d, cells that are probably immigrated leucocytes. The cells of the discus have not yet been minutely studied; those next the ovum are cylindroid, and radiate around the zona, constituting thus the so-called corona radiata of authors — compare Fig. 34. The cells of the outer layer of the discus are more rounded in form ; it is, of course, probable that the two forms of discus-cells resemble the cells of the granulosa in actual shape.

Fig. at— Orary of OiMflan (olli

Just before the primary follicle changes into the Graafian follicle the ovum, at least in man, has attained its full diameter, but still contains no yolk (deutoplasni) . At this time there appears a clear, delicate membrane close around the ovum, separating it from the cells of the follicular wall. In the Graafian follicle this membrane steadily grows until it attains a diameter of 20-24: // ; it is called the zona radiata or pellncida: its structure is described in the subsequent section on the full-grown egg-cell.

The first yolk-grains appear in the human s[)ecies when the zona pellucida has attained a thickness of 1 /t or more, and are situated always in the centre of the egg-cell (Nagel, 88.1, 38o, 380). In other mammals they are said to appear earlier. The yolk-granules must be considered as the direct products of the vital activity of the eggcell itself, and in my judgment there is no sufficient basis for any other view. Various hypotheses as to the origin of the yolk-grains have been advanced. Thus Waldeyer, 70.1, has maintained that the grains are produced by the cells of the follicle, find are transferred from them across the zona into the ovum. It is not impossible that very small young granules may arise in the follicular cells and be transmitted along the fine processes by which the cells are connected through the zona radiata with the ovum, and that these granules subsequently grow within the egg-cell, as Caldwell, 87.1, asserts is the case in monotremes and marsupials. Caldwell's statements are so aphoristic that the question must remain unsettleil until more fully investigated. Lindgren, 77. 1, asserts that the cells of the granulosa immigrate through the zona to form the yolk-granules; his observations were made on ova which had already been somewhat macerated, and which had the processes of the follicular cells swollen in consequence. That the yulk-grains are produced by the graflual enlargement of small ones has been shown by Sarasin's researches on reptiles, 83.1. He found in Lacerta a central area of small granules which gradually enlarge; this area {Herd der Dotterhildunq) persists even after the embrj^o has appeared, and the egg increases in volume and weight after the segmentation has begun.

The characteristics of the human yolk-grain have not been accurately investigated, nor have those of any of the higher mammalia been studied carefully. In the human ova the grains are I li or less in diameter; highlj' refringent and of various kinds. In a sheep's ova Bonnet, 84. 1 , found small granules, fat-globules in considerable abundance, p, lt8, and larger granules which stain with eosiue, I. c, p. 18;j. Their accumulation continues centrifugal ly, forcing the nucleus of the ovuui to au eccentric position; when the maximum of the vitelline dejwsit is reached tbepi is only n verj- thin layer of protoplasm around the outside of the egg-cell, Fig. 34, and a court of protoplasm aroimd the nucleus. This disposition is particularly well shown in the ova of the monotremes and marsupials. See Caldwell, 87. 1, PL XXIX., Fig. 5. The cortical layer is readily distinguished in fresh ova, but in hardened Bpecimena is quite or wholly indistinguishable. In the ovum of the placental mammalia the yolk never attains a great development; but in most vertebrates the granules gradually eiiJarge, and in some cases they are quite big. When they are thus developed it is ea,sy to see that thej- are of various sorts Thus in the hen's ovum there are two principal kinds of yolk grams the mellow and the white. The yellow grains are spheres of from ^5 f to 100 /* in diameter, filled with numerous mmute highl} refractile granules; these spheres are very delicate, and easily dPStroye<l by crushing. When boiletl y ■^^^^'^V: or otherwise hardened in situ, they assume a

dft \ \ pol^heilral form from mutual pressure. The

,"^i \ white grains are vesicles, for the most part smaller

(i / to 7.5 />) than the spheres of the yellow yolk » ith a highly refractive body, often a.s small as 1 /t m the interior of each. Thei-e are also larger ipheres, each of which contains a number of spheiules similnr to the smaller vesicles. The yolk plates, or plagiostomes, which consist principally of lecithin and nuclein, are not present in the younger ova, but are present in great nuin•Tm' la In"""!*'*" "' bersin the full-grown ones; they are oval, barrel Buff^" ' 2'."1Iimfoat.^ 8hai)ed, or rectangular Ixidies, with rounded eor^^-uuvitfUB After Bai- ^p,.j^ j^,] e<Igea ; tlic siirface, esiK'cially in the larger plates, shows a fine transverse striation, corresponding to the laminate structure of the grain. As no thorough comparative investigation of the yolk -granules has l)eon made, it is not worth while to enter into further details.

Besides the yolfc-gniina there may also he present one or several large masses of nutritive material, such as the " nil-globules " of many teleosts, or the so-called yolk-nuclous. The " oil-globules " are produced by the liquefaction of the yolk, and arc not oily. The yolk-nucleus has lieen descril>ed by Balbiani in the Araclmida. The eggs of some spiders contain. l>esides the nucleus, a second body (Fig. Xi, k), of auout the same size as the nucleus, solid, resistant, and exhibiting indications of a series of concentric lamina? ; this is the so-called yolk-nucleus, and is probably only a specialized form of deutoplasm, and might be compared, for instance, to the four large oil-globules described by Sjiengel in the eggs of Boiielliik viridis.

A yolk-nucleus has hiucq \yeen recorded in the ova of various vertebrates; thus Schiitz ('* Ueber den Dotterkeni," Diss. Inaug., Bonn, 188'2) found in the ovarian ova of the pike, in September and October, a round or oval body, not sharply delimited, clear, and more homogeneous than the protoplasm, and which increase<:l in size with the growth of the egg. A yolk-nucleus, c<jnsisting of an accumulation of larger and smaller granules, has also been observed in the frog and newt (O. Schidtze, 87. 1), but is a})parently wanting in Bufo and Bombinator (Ootte).

The amount of yolk varies in different animals very greatly, and determines, ap]^>ai*ently, the size of the ovum. It has been observed that the process of segmentation varies according to the amount of yolk, and this has led to the arbitraiy division of ova into men Mastic mid holobhtstic (see Segmentation of the Ovum). The yolk usually, perhaps always, leaves a peripheral layer of protoplasm free. In all vertebrate and in some invertebrate ova this layer of protoplasm is thickened, often considerably, around one pole of the ovum, which is then distinguisheil as the animal pole, the opposite pole being called the vegetative. These are old terms, which have come down to us from the time when the ectoderm, which is produced during segmentation, principal!}' from the substance of the animal pole, was called the animal layer and the entoderm the vegetative layer. It ia at the animal |x>lo that the exti-usion of the ix)lar globules under normal conditions invariably takes place.

The Graafian follicle grows very much more than the ovum, until it becomes a large cyst. Fig. 3*2, the position of which is marked by an external protuberance on the surface of the ovar3\ To the deep wall of this cyst is attached the discus proligerus with the ovum, which is now nearly full grown. The stigma is at the protuberant point of the follicle, which is covered by very little ovarial tissue, so that there is a ver}' thin wall only separating the cavity of the follicle from that of the al)domen.

The degeneration of the* Graafian follicles with the contained ovum occurs normally in the ovary ; but as the process has no direct interest for the embryologist, it will suffice to refer to Fronnnann's very admirable summary (Eulen burg's "Real. Encvclop. Heilkunde," V., r)02-r,()4).

Full-grown Ovum before Maturation. — The full-grown human ovum is distinguished among mammalian ova for the clear development and ready visil)ility of all its parts — a peculiarity due chiefly to the small amount of the yolk and fewness of the fat-granules it contains. Fig. o4: represents an oviun from a nearly mature Graafian follicle of a woman of thirty years ; the si)ecimen was obtained by ovariotomy and examined and drawn in the fresh state, being kept in the liquor follicle. This specimen- gave the following measures- The diameter of the whole ovmn, including the zoua radiata^ 1(»5-170 /<; thickness of the zona, '20-24: /'•; peri vitelline fissure, 1.3 ;i; the clear outer zone of the yolk, -A-O //; the })roto})lasmic zone, 10-^1 fi ; the deutoplasm zone, 8*2-87 // ; the nucleus, '25-11: //. the corona radiata, ror. 7-., exhibits the features already described. The zona pellucida, Z, shows a distinct radial striation ; this is probably due to the presence of minute ix)re canals running through the zona, and which, at least in early stages, give passage to processes of the tells of the corona radiata, which unite with the ovum. These processes have not yet been oteened in man in an altogether satisfactory manner, and indeed Nagel. 88.1, 4ii-^, expressly denies their exif^tence, as well as that of the pore canals. The processes are, however, readily seen in the lower vertebrates, in the monotremes and marBupials, Caldwell 87 1 and have been observed in the placental mflmmaha Fig 35. Hence it seems probable that they are present in man at least wlile the o\uni i a growing, th ugh thej may be obliterated at the stage we are now cont-idenng Several observers record dumb-bell cells" * with the thm portion of the cell pa s&mg through the zona, \li and nekn bljmgontheoutside tl e otl er on the inside, of the zona compare H. Vir^,p chow 85 1 But apparently Afutr such observations have been made solely on ova that had been somewhat macerated, and therefore the " dumb-bell cells" result probably from post-mortem clianges, and cannot be interpreted as by Lindgreii, 77. 1, to prove the actual normal passage of cells of the discus proligerus through the zona. The zona has no micropyle or special open channel for the entrance of the spermatozoon. For additional details, see the following section on the envelopes of the ovum.

The ovum proper is sejiarated by a narn>w fissure, p v, the perivitelline space, from the zona, within which it lies free and loose, so that when a freah sijecimen is exaniine<l the same side of the ovum — that containing the nucleus, which is the lightest i»art — is always found nppei-most.

The ovum has no vitelline membrane, according to Nagel, 88.1, 405 ; but in several mammals such a membrane has been described, appearing as a thin, delicate line abont the time the ovum matures, Fig, H5, v.m. The botly of the ovum may bo divided into an inner kernel containing the yolk-granules and an outer protoplasmatic zone, of which the very outermost thin layer is clear, and therefore more or less differentiated from the broader, deeper laver, which is granular and constitutes most of the zone. Frommann, &9.1, has shown that many of the granules in the ova of the sea-urchin are part of the protoplasmic reticulum: in the living egg they are incessantly changing in shape and in their connections, even disappearing and reappearing; the disappearance Frommann terms liquefaction, the reappeanince a new formation. It seems ht me possible that the changes !*e«'ii are probdbly in part effects of contraction in the reticulum. the nucleus is nearly spherical, always eccentric in position, and has a nucltxihis whicli in the fresii specimen shows Hmueboid niovt'inentj' even at oniinary summer temperatures for several hours after removal from the ovary, Xagcl, 88.1, 40*. In hardened specimens the nucleus shows its retictiliim, as alreadj- descrJbetl.

i. procopUstn

  • Nagvlielleu, Spun

I, ZwIlliDgszEllea. or Hutel/elleD of <irrniaii wHten.

In certain ova there has lieen observed a special band of prot«plasm lea«ling from the surface of the ovum to the egg nucleus. This ia found in the ovum of Petromyzon, having been first described and figured by Calberia, 78.1, who, however, erroneously designated the nucleus as the female pi-onucleus, and interpreted it aa the pathway iwrformed for the passage of the siwrmatozoon — an error which Boehm bjis corrected by showing that the true pi-onncleus is formed later. As shown in the section on impregnation, p. (iii, the pathway of tiie spermatozoon can be traced in certain amphibian ova.

Peculiar names have been applied to the nucleus and nucleolus of the ovum, and are still in general use. The nucleus was first discovered in 18:i(i by ^_ Purkinge ("Sym- ~ bolaj ad ovarium historiam,"lHf(i)in hiidh, and by CV)ste (lft3T) in mammals, and beiiune known as the lesi(ulaqenniuutivu, {F'lrkiiijf'xihfs ifUiictieii. or germinal vesicle). The nucki>Ius was first descrilxtl in is:i5 by R. Wagner, 36. 1, and became known as the germinative or Wagnerian spot ( Wdfiiier'mhfi' Fli-ck). It was not, however, until 1M!I tiiat Theixlore Hchwjinn for the first time interprettxi the ovum as a cell; but l)efore then the terms germinal vesicle and germinal spot had established themselves, and since then they have remained 111 general use.

The Envelopes of the Ovum.— The eggs of different classes, anil even s[>eoies of animals, are, as is well known, extremely unlike in appearance. The dissimilarity refers chiefly to size, to the charactei ot the yolk, and the nature and numlter of membranes or other envelopes, liy which the ovmn or egg-cell proper is surrounded. Thus in the hen's e^ the yolk alone represents the part corresponding to tile egg-cell, while the white of the egg and the egg-shell are only secondary envelopes, tlit; foniior serving to nourish, the latter to protect, the si^-called i/olh. \vliich is the essential part, the true egg The various envelopes which eggs ever have may be classed under tour categories; Fir.if. a very thin and delicate one, the proper membrane of tbiM-cll itself, and which ought always tti be distinguished as the vitelline membrane; second, the ovarian envelopes, which are secreted around the egg-eel! by the tissues of the ovarj'; third, the envelopes secreted by the oviduct, which may form a coating of nutritive material, or a protective shell, or both, as in the hen's egg, of which the nutritive white ia secTeted bj' the upper part, the calcareous shell by the middle part of the oviduct; fourth, coverings secreted by accessory glands, such as the slime in which the eggs of snails are embedde<i, or the tough capsules in which leeches lay their eggs. By adhering to this classification it is possible to find one's way through the labjTinth of special descriptions. It is impossible to review here the manifold variations in the ovarian coverings of animals, and we shall attempt only to describe those of the higher forms.

All vertebrate ova probably have two enveloiies . first, a verj- thin inner one, the vitelline membrane proper; nerond, a thicker ovarian membrane, known as the zona raduita or pell xvida. The vitelline membrane is described by Heape, in the mole, as a verj- thin but distinct membrane (Fig. 3j, r. in.), immediately against the yolk, separated by a narrow space from the zona , it is to be regarded as a product of the ovum itself. It appears a short time before the ovum matures, and is most distinct at the time of the formation of the polar globules ; its fate during segmentation has not been ascertained. The so-called vitelline membrane (Dofterhavt) of amphibia is really the homologue of the zona (Frommann). Considerable doubt in regard to the. presence of this membrane in vertebrates, and especially in mammals, has been expressed by various writers, but its existence seems to me to have been sufficiently demonstrated. It was first described by Reichert in 1841, and i^ain by H. Meyer in 1842. In recent years it has been redescribed by Ed. van Beneden, by Heape, and others. Balfour, in his " Embryologj-, " pronounces in favor of its occurrence. The zona radiota (pellucida of C. E. v. Baer), Fig. 35, Z, is a membrane, usually of ^^^- — -- ^ considerable thickness, which can be dietin ■' guished around the ovarian ovum quite

// . early, being at first very thin, but gradually

mcreasing in thickness until it attains in man a diameter of al>out 20 n in the mature ovum. In the pig tiie diameter becomes 7 ^ ' ' ^ to il .«; in the sheep, 7 to 12 ,"; in the cow, 7

—"'^ ' to S :•■ (Schulin) ; in the molo, 8 to 11 /i, according to Heajje. In the mature o\-um it IS a tough, clear, glistening memltrane. verjFig. 30 (iMiiii i.f as.'niir rcsistant to acids, and aoluHo in alkalies only Afi°r o-'hi'SIvIk"" '" "'" with difficulty. It is pierce<I by numerous radiating [wrcs, which produce the api)earance to which the tenn zowt rodUita refers. These pores were first ol)served by Johannes Midler and Remak in fish eggs,* and they have since bfeen observed in the ova of many other vertebrates, including several species of mammalia. It is pmbable that they always exist, despite the doubts expressed by Schulin, Liiid

gren. Von Sehlen, Nagel, and others. While the ovum is still in the ovary it is surrounded by the cells of the discus proligerus ; these cells send processes through the pores of the zona (Fig. 3o) . It is now commonly supposed that these pr<x*esses are channels of nutrition for the ovum. Ihe zona is somewhat granular in its outer portion, next the cells of the corona. Balfour has suggesteil, not ver}" plausibly, I think, that the granular ix)rtion does not belong to the zona, but represents the remains of a hypothetical primarj' vitelline membrane, within which the zona projjer arose subse<iuently. Another very hy|x:>thetical homology is suggested by Caldwell, 87.1, who finds two membranes around the ovarian ovum of marsupials ; the inner membrane resembles the ztma pelliicida, and is termed by Caldwell erroneously the vitelline membrane; the outer membrane is the proalbumen, which, during the jjassago of the ovum through the oviduct, swells up and becomes the albuminous envelojx? of the ^Kg' Caldwell homologizes the inner clear layer of the z<jna of the placental mammals with the zona of marsupials, and the outer granular layer with the proalbumen. In Petromyzon (Boehm, 88. 1), as in some teleosts (J. Brock, TS.l), there are two ovarian envelopes which are quite probably homologous with two envelojx^s found in marsupialia, but that the zomi of the placentalia represents two envelopes united in one is, at least, very uncertain. It seems to me that the zona nidiatu is to l>e regaixleil as a m<Klified intercellular substance, and that the processes going through itsjxjres are to l)e homologized with the ordinary intercellular pR>toplasmic bridges of epithelial cells.

Heape thus describes the jwres of the zona in the mole: "The radially striated api^earance of the zona has long l^een shown to be due to a vast numljer of tin ^ canals i)assing radially through it. The canals, I find, o{)en on the inner side of the zona by a slightly dilated mouth, while on Jthe outer side of the zona they communicate with the exterior by a considerably wider opening, Fig. 35. Into the external openings of these canals I have been able to ti*ace prolongations of those cells of the discus which are immediatelv in contact therewith. Fig. oh^ and there appears to me no nx)m to doubt that the contents of these follicular cells an: thus rendered available for the nutriment and growth of the ovum."

The term in icrop!/le is ust^l to designate a jiassage through the enveloi)es of the ovum, which series to admit the spermatozoon. The micropyle is j)rc»sent in many invertebrate ova, notably in those of insects, and may have a quite complicate<l structure. In the vertebrates it is very rarely found, having liet^n thus far j)ositively demonstrated only in certain tel«.Mjst eggs. Callx^rhi, 78.1, affinned that a micropyle was pres«*nt in Pt'tromyzon ; but Boehm. after a later and more thorough investigation, 88.1, expressly denies its existence, Kupffer and BentK*ke. 78.2, having prc»viously shown that the s|)ennatozoa |x.»netrated the lanipn»y ovum at several jnnnts. Sundry authoi*s from time to time have asserttnl that a microj)yle was present in the mamamlian ovum, but tin* evidence against it seems to me conclusive.

The corona radiata is the name given to the env»*loi)e of cells of the discus jiroligerus, which adheres for a short time to the zona radiata when the ovi:m is discharged from the Graafian follicle. The corona may be represented only by a few patches of cells, or may be a complete envelope; in either case the cells are entirely lost soon after the ovum begins its descent through the Fallopian tube. The egg of Lepidosteus has two envelopes; the outer one is homologized by Beard with the corona radiata, but E. L. Mark, 90. 1 , denies this homolog>\

The disappearance of the zona has been specially studied by Tourneux et Hermann (C. R. Soc. Biol., Paris, 1887, p. 49), who found that it could be distinguished in rabbits' ova of ninet>'-five hours, but not in those of one hundred and sixteen hours. According to Hensen the zona in guinea-pigs is ruptured, and the ovum escapes during the descent through the oviduct.

Polarity of the Ovum.— The mature egg-cell has a distinct axis, the two poles of which are unlike in character, while around the axis there is a complete radial symmetry so far as known. In my opinion the essential difference between the two poles is that the nucleus is nearer one than the other, and consequently the protoplasm of the egg-cell is more concentrated at one pole than at the other ; for, as is well known, the nucleus usually has an accumulation of protoplasm around it. The eccentric position of the nucleus is, I think, probably universal. Curiously it is frequently stated that the nucleus lies in simple ova in the centre,* and the notion is prevalent that the accumulation of yolk is the cause of the eccentric position in certain ova. This notion is not quite correct ; on the contrary', we must assume that the position of the nucleus causes the eccentricity of the yolk material. There is unquestionably a strong tendency for nucleus and protoplasm to keep company ; thus we see when cells are connected with one another by protoplasmatic bridges, a main cell-body around each nucleus. Again, within single cells, the protoplasm often forms a court around the nucleus and a looser network throughout the rest of the cell ; in ova with incomplete segmentation each nucleus is imbedded in its special accumulation of protoplasm; it appears to me, accordingly, that the disposition in the egg cell is only a special instance of a more general principle.

The eccentric position of the ovic nucleus is due to as yet unknown causes; but being given it determines the accumulation of yolk-grains at the opposite pole; it will be remembered that in the developing egg-cell the nucleus becomes eccentric liefore the yolkg^ains appear. The amount of yolk undoubtedly affects the degree of the nuclear eccentricity. The nucleus reigns over a comparatively small territory, within which there is no, or but very little, yolk-matter developed ; in all vertebnite ova the perinuclear protoplasm touches the vitelline membrane and marks externally the site of the nuclear or so-called " animal" pole. In the rest of the egg-cell the yolk-grains may be freely developed, and as they increase in number and size there is a corresponding distention of the region of the cell which they occupy. This distention may go so far that, as in the birds' ovum, the perinuclear territory is minute compared with the great bulk of the deutoplasmic territory, and consequently the nucleus lies far away from the centre of the ovum.* The yolkgrains centre about the pole opposite the nucleus, which might therefore be called the vitelline or deutoplasmic pole, though it is still generally known by the inappropriate name of vegetative pole, which has come down to us from long ago.

  • For example O. Hertwijr. 1H8H. 1. p. H. says ' das K«Minblaschen luj;t*rt >?ew(>huiich in der Mitte des Eies, yet lii8 own flj^res correctly represent it as eccentric.

F. M. Balfour, in his '* Comparative Embrj'^olog}'," divided ova into three classes, as follows: 1st, alecithaU without any deutoplasm; 2d, telolecithal^ with the deutoplasm collected opposite the animal pole; 3d, centrolecithal^ with the deutoplasm in the centre surrounded by a cortex of protoplasm. It is j^robable that all ova are telolecithal in the sense that they have a nuclear pole, and that the yolk-matter is developed away from the nuclear pole. The alecithal ova are those in which the nuclear eccentricity is at a minimum; the cent roleci thai ova, which occur only among invertebrates, are likely to prove to be really telolecithal. All known vertebrate ova are telolecithal.

The polarity of the ovum dominates the pr(X*ess of the ripening of the egg-cell, and has a very imjwrtant influence on the process of segmentation after impregnation. The extent of this domination has been thus summarized ])y E. L. Mark, 81.1, 515: '*The migration of the genninative vesicle toward a definite iH>iut of the surface; the radial jxjsition assumed ])y the maturation spindles; the waves of constriction which precede the formation of the jx^lar globules, and the inequalities in the sizes of the latter; the union of the pronuclei at a ix)iut nearer the primary than the secondary pole, and the consequently (?) eccentric position of the first segmentation spindle; the appearance of the first segmentation furrow earlier at the primary than at the opposite ix)le; the formation of pseudoixxlia-like elevations, often most conspicuous at the primary pole ; the accumidation of finely gnmular protoplasm at the secondary \hAq after the elimination of the polar globules ; and the appearance of ' polar rings ' and * ring rays (Clepsine) at both ends of the primitive axis, are all indications of a |)olar differentiation of the egg. "

The polarity of the ovum also evinces itself in the difference of the specific gravity of the two jx)les ; usually, as in mammals, birds, amphibians, many fish and invertebrates, the deutoplasmic pole is heavier, and the ovum always presents the animal pole upijemiost as soon as it is left free to turn ; in the ripe mammalian egg the yolk has ro<)m to turn within the zona : hence when the fresh ovum is examineil under the micrusco|)e, the animal pole is toward the ob8er\'er and the eccentric position of the nucleus cannot be obser\'ed. In various |)elagic teleost ova the animal ix)le is the heavier, and the embryo develops accordingly on the under side of the egg.

Maturation of the Ovum. — The term maturation is restrictetl by usage to the series of phenomena accompanying the expulsion of the |x>lar globules which (xcui-s after the egg-cell has attained its full size, and just Ix^fore or just after the separation of the ovum from the ovar>\ A jxilar globule is a small, nucleated mass, extruded fnjm a fully-growni egg-cell.

When an ovum i< al)out to mature its nucleus moves nearer that point of the surface which may be regarded as the centre of the animal pole, and there also occurs a contraction of the vitellus. The centrifugal movement of the nucleus was first observed by Von Baer, 27.1, 21>, in the hen's egg, and has since been seen by very numerous observers and in very numerous species ; it must, therefore, be considered as an unvariable phenomenon. Concerning the force which moves the nucleus we have no definite conception ; for discussion of the question, see Whitman, 87.3. The contraction of the yolk is prolxibh' also a constant phenomenon ; it is apparently effected by the expulsion of fluid from the protoplasm, so that a clear space separates the zona and yolk. The observations have not been , coUateil yet on this point, and it is impossible to state whether there is a constant rule as to the extent and epoch of the contraction.

  • Ryd»rr lia^ |iiilWi«hH<l a s«*ziii-i^>iMiIar iliflcusflioa of nuclear displaiN^uu^nt. H3. 1.

After reaching the surface the nucleus as such disappears. This fact was known to Purkinje, 30.1, 15, the discoverer of the nucleus, and has been shown to occur in all eggs which have been accurately examined. K. E. von Baer maintained both in 18*27 and subsequently, 37.1, 4 and 1), 37.1, 28, 157, 297, the opinion that the disappearance of the germinal vesicle was connected with the maturation of the ovum — a conclusion which is now established beyond question. Reichert in 184G, 46.1, 190, 205, maintained that the disappearance was the first result of impregnation, and in this error he has had several followers (A. lliiller, Haeckel, Biitschli, and others). In birds the nucleus assumes a very large size, and migrates to the surface of the ovum, when it disappears as shown by Oellacher. M. Holl, 90.1, records that in a newly hatched chick the ova measured about 14 /i X 9 /*, while an ovum nearly ready to leave the ovary measured 40 X 35 mm ; in the former the nucleus was about 9 //, in the latter 315 x 117 /i in diameter. No polar globules have yet been observed in birds, though we must assume that they are formed.

The disapjiearance of the germinal vesicle is only apparent, not actual, being in reality a metamorphosis. It is probable that the first step is the discharge of nuclear fluid {Kernsaff) into the surrounding protoplasm. This is indicated by two apjiearances — 1st, the shrinking of the nucleus, the outline of which becomes shrivelled ; 2d, a clear space which arises around the nucleus. The shrivelling of the nucleus has l)een observed in several mammals (Van Beneden, Rein, Bellonci, Tafani) in various vertebrates — as, for instance, in teleosts by Oellacher, 72.1, 3, in Amphibia by O. Schultze, 87.1, and in manv invertebrates, ^.7.,Serpula bv Schenk (Sitzber. Wien. Akad. LXX.Abth. 3, 291-294, 1875), in Hydra by Klemenberg, 72.1, 42, in Asterocanthion by Ed. van Beneden, 76. 1 . The clear perinuclear space has l)een noticed es]Decially in Anura by Gotte, 75.1, 20-22, and O. Schultze, 87. J, 217. The second step is the dissolution of the membrane of the nucleus, so that the nuclear contents are brought into direct contact with, and partly mix with, the cell-plasma. Verj" likely this mixture of nuclear and cell substance is, as O. Schultze suggests, 87. 1, 215, one of the essential factors of maturation. The dissolution of the nuclear membrane has been found to occur in so many species that we may safely predicate it of all. We now find the contents of the nucleus lying together in the centre of the protoplasm of the animal jK)le. The contents themselves are altered in

ctiaraet«r, the most noticeable chuiige being the breaking up of the cbrumatin into separate granules in niamiiialu the formation of the granules by the cleavage of the nucleolus »K.cur3 aftei the nucleus has begun its inigrati<)n (van Beneden Bellonci, Tafani), m Am pliibia the nucleus becomes multinucleate duimg the earlj growth of the ovum. The achromatic substance or reticulum of the nucleus appears as threads often very difficult to recognize.

The threaiis and granules proceed to group tbeinselves into a iipindle-Hhap<;d bod\ , the so callotl nuclear spindle (AVrw.s/x/K/W) which , lies more or less nearly in tlic radius of the ovum and has one of its ends cI<iso to the sur face of the yolk. Fig. jS, up. The achromatic threads run from jxtle to polo of the spindle, the chn»natin granules lie in the centre of the spindle in one plane and produce the appearance of a transverse l>and or disc (Straas- HtemopJ «' burger's Kernplatfc) ; each chromatin granule J^ KHj""^ is iLssociated with one of the spindle-threads, "'kEach pointeil end of the spin<lle lies just within

a rounded clear space, fiinn which, and not from the end of the spindle, radiates threads in the yolk, whence results a figure like a conventional sun. The %vhole spindle with the two suns has been named the amphia.tier. As amphiasters occur in connection with ortliiiar>- iiidirt>ct cell-division the distinctive term arvhiamphiaster lias been proposed for those concerned in the production of the polar globules. Sometimes as in Liimix, Mark, 81.1, the astral rays are not straight, but cui^ed as in a turbine. In amphibian ova only a portion of the granules enter into the formation of the chromatin, while the majority of thcni are mingle<l with the yolk (0. Schultze) ; it is possible that this modification is conncctetl with the bii^ amount of yolk and will l>e found in other vertebrate ova. In the ova of mammals (all?) the chromatin enters into the " Kentpltttfe." The shape of the spindle varies, as does also the distribution of the granules of the nuclear plate, thus. In the guineapig, the ends are pointed and the threads are straight, so the outline of the spindle is like a diamond ; in the bat the spindle is barrel-shaped and the threads are cur\-e(l. In certain, possibly in all, cases the spindle, when first formed, ties obuii-. /. y Aru-ro MrnniK liijuelv. and subsequently becomes erect

to the surface, iis Whitman observed in the leech (Clepsine, 78.2) ; for further reference, see O. Schultze, 87.1, 21!t-3'-il. The reason for the obliquitj' and the following erection is imknown.

The next changes may be followed with the help of Pig. 3(t. The spindle, driven by an undiscovered power, continues the centrifugal movement until it is partly extruded from the e^, as shown in the figure ; the projecting eiiil is enclosed in a distinct maiiS of protoplaBtn, p.g., which is constricted around its base. The fragments of chromatin have each divided into two, and one-half of eacli fragment has moved toward one end, the other half toward the other end of the spindle. The lialf-fragments of each set move together, hence there seem to be two plates within the spindle. The translation of the groujjs of chromatin grains continues until they reach the ends of the spindle; the achromatic threads then break through in the middle. Thus the original nucleus, or at least jmrt of it, has been divided. There are now two maases of nuclear substance — one in the ovum, the other in a little appendage to the ovum; this appendage is the first polar globule; its nuclear substance does not develop into a complete nucleiis.

The remnants of the egg-cell nucleus within the ovum undergo further changes. Usuallj-when the amphiastral (indirect or kinetic) division of a nucleus is over, the separated nuclear masses resume the structure of a normal resting nucleus ; but in the ovum, as Platner, 89.1, has especially noted, the nuclear remnants change directly into a second spindle, which lies as did the first within the protoplasm

of the animal iiole, and likewise gives rise to an amphiaster {second

^ arrhiamphiaster, zveifes liicktiingsspin ^- ' '.%- del). The second spindle even more clearly

'^^"~33 ^^^^ ^^ fi'^' liss been observed to occupj'

ii~~ _f an oblique ]K>sition, as in mammals (Bellon ci, 85.1), oreven parallel with the surface,

(T\ ' as in amphibians (0. Schultze, 87.1) and

^ certain Crustacea, Weismann and Ischika ^^^' wa, 88.4. This spindle pro<luces a second

"^ polar globule in similar manner to the first;

■^ the globule is somewhat smaller than the

Fio SD— Ovum or NVph^iisCtt first. and is at least sometimes connected

^^ii?^ibu'i'«"/'™»ip>^- "*'^ ^***^ *® ^^^* globule and with the

nuc'ir-us : IN. nmi'e ' pn.nucieus. ovum. Sometimes the first globule divides

ter-.. erKrig. j^^^ ^^,^ pj^ .y^^ ^^ ^^j they may remain

connected together. the oonnectitm of the globules with the yolk persists for some time, and in the case of leeches is not dissolved imtil segmentation begins.

The polar globules ultimately disappear — how is not exactly known. That they take no part in the further historj' of the ovum may be considered established ; for they break off and may often be seen in mammals knocking about within the zona, while the ovum is developing after impregnation, and they then present a hyaline appearance, as if slowly degenerating.

The number of polai" globules, as Weismann and Ischikawa, 87.2,88.4. first explicitly demonstrated, is two. According to these authors, 88.4, 590. two polar globules have been shown to occur in 8 species of crelenterates, .'i of plathelminths, fi nemathelminths, 1 zephyrean, 10 annelids, 5 echinoilerms, 2'2 niollusks, (i tunicates, 1 bryozoon, 15 crustaceans, insects, II vertebrates. It can hardly be doubted that two polar globules are necessarj" for the complete maturation of the ovum, juid that until they are formed impregnation cannot take place. On the other hand, Blochmann discovered that in a parthenogenetic ovum there is only one polar globule formed, and Weismann and Ischikawa, 88.4, have shown that this is true of many and presumably of all parthenogenetic ova — that is, of ova which develop without fertilization. For the theoretical consideration of the polar globules, see below.

The polar globules appear to have been seen as long ago as 1837 by Dumortier in gasteropods, and in 18i0 by the elder Van Beneden, and in 184*2 in the rabbit by Bischoflf. Fr. Miiller observed them more carefully in 181^8, and detecteii their constant relation to the planes of segmentation, and gave them the current German name of Richtungskorperchen. Robin, in 1802, termed them globules polaires^ which, translated, liiis become the accepted English designation. Biitschli, 76. 1 , in 1 8T(), first led the way toward a correct conception of the origin of the globules, and about the same time came the independent researches of O. Hertwig, whose able memoirs, 76. 1, 77.1, 77.2, 78. 1, have formed the basis of all subsequent work. These were soon followed by the investigations of Fol and many others. From these studies we possess a tolerable general conception of the origin of the ix>lar globules, but the comparative study of the details and variations remains for the future.

After the formation of the second polar globule there is a small group of chromatin elements and achromatic threads, which, since they have Ixjen halved twice, represent approximately one-fourth, not of the whole egg nucleus, but of so much thereof as entered into the formation of the first jjolar spindle. The nuclear remnant lies close to the animal pole and in the cleiir protojJasm ; it is the so-called female pronucleus^ the history of which varies according to the species of animal. Three tendencies are known to affect the pronucleus — namely, to move toward a central position in the ovum ; to unite with the male pronucleus as soon as that is formed out of the s))ermatozoon, which enters the ovum to fertilize it, and to assume the character of a membranate nucleus. As the time of the formation of the male pronucleus is variable, the other tendencies l)eing more constant, the exact history of the female pronucleus may be said to depend principally upon the appearance of the male pronucleus. The earlier that event, the less does the female proimcleus move centripetally, and the less does it assume a nuclear form. In mammals as in echinodenns, the female pronucleus acquires a membrane, and lies, when the si>ermatozoon enters, near the centre. It is very much smaller than the egg nucleus (compare Figs. 30 and 39), and is remarkable for its homogeneous apjx^arance and the absence of nucleoli. In other animals, e.g, Petromyzon, it is merely a cluster of granules. For further details as to the pronuclei, see the following section on impregnation.

The time when the polar globules are formed varies, and according to the animal may be before or after the egg-cell leaves the ovar}'. In placental mammals the maturation always begins, so far as faiown, in the ovar^% and may be completed there, or it may go on in the Fallopian tube, as Tafani, 89.1, 114, states is the case in white mice. Our knowledge of the maturation of the mammalian ovum is very imperfect, and rests almost exclusively uix)n observations on bats and rodents (rabbits, mice, rats, and guinea-pigs), and even on these the observations are very incomplete. See Ed. van Beneden, 80.1,Van Beneden et Julin, Rein, 83.1, Bellonci, 86.1, Tafani, 89.1.

III. Ovulation

The process of ovulation, or the discharge of the ovum from the ovary, has to be considered from both the morphological and physiological standpoint. The discharge results from structural changes in the Graafian follicle, and these changes continue after the departure of the ovum, transforming the Graafian follicle into a so-called corpus luteum. Concerning the physiology of ovulation we know almost nothing beyond the coincidence in some species of mammals of the time of the bursting of the follicle with certain periodic changes in the uterus.

Ovulational Metamorphosis of the Graafian Follicle. — The mature follicle measures some by 12 millimetres, being elongated in the same direction as the ovary, but its dimensions are variable. The granulosa is ver}' thin, and its cells show signs of a fatty degeneration. It is probable, I think, that this degeneration progresses to a considerable extent, and involves the loosening of the granulosa cells ; for loose cells, granules, and fragments are found in the liquor foUiculi. The cavity of the follicle is very large and filled with the fluid, which seems to be under pressure, since it spurts out with considerable force when the follicle is pricked. It is to the pressure of the liquid that Coste attributes the rupture of the follicle. Waldeyer in Strieker's "Gewebelehre," p. 571, describes a growth of the wall of the follicle, w^hich causes it to form a series of folds which protrude into the follicle; this ingrowth prcxluces the force that exi)els the ovum. Unfortunately, Waldeyer does not state on what animal his observations were made; they certainly do not apply to the human species, for there is in man no considerable growth of the follicular wall until after the rupture. The stigma becomes, meanwhile, very thin, and finall\^ breaks through. Coste's observations, 47. 1, 172, on rabbits eight or ten hours after the coitus, showed that the rupture is not abrupt but gradual, the membranes of the follicle giving way first, and the peritoneum a little later. When the stigma breaks, the liquor foUiculi, together with the ovum surrounded by the discus proligenis, escapes, and the ovulation, seiisn strictn^ is completed. The fate of the cells of the tunica granulosa is uncertain, though Benckiser, 84.1, has shown that in the pig they disappear at the time of or soon after the rupture. I consider it probable that they are lost in man at the time the o\'um escapes ; it may be that they degenerate; it must be mentioneii that some writers maintain that the granulosa persists and takes part in the further metamorphoses of the follicle. At the time of the rupture there occurs a hemorrhage of blood into the emptied follicle, and this blo^xl forms a clot which fills up the entire follicle, and is knowTi as the corpus hemorrhagicnin. The hemorrhage may vary in amount or even be wanting altogether, as Benckiser, 84. 1, found in 8 cases out of 100 in the pig. Leopold expressly states, 83.1, that when the follicle ruptures at the menstrual period it is always filled and distended by blood filling it, but when the rupture occurs in the intermenstrual period the hemorrhage is small or altogether wanting; the presence of blood is therefore not indispensable to the formation of the corpus. When the follicle contains no blood it is filled with a whitish coagulum of unknown origin (Coste, 47. 1 , 1. 245) . The coagulum, whether of blood or not, is rapidly penetrated by tissue which grows into it from the wall of the follicle, accompanied by numerous blood-vessels; the cells of this tissue have two principal forms, His, 66.2, 18G-187: first, spindle-shaped connective- tissue cells, which lie principally around the blcxxl- vessels; second, large cells, which contain granules i)f a pigment, called lutein from its color; these cells are the luteincells, and are the characteristic elements of the metamorphosed clot, to the margin of which they impart a bright yellow color, whence the name corpus luteum. The ingrowing tissue is derived from the inner layer of the theca folliculi. That the blood-vessels and spindlecells have this origin has long been the generally accepted opinion, and though the origin of the lutein-cells is under dispute it is probable that they arise exclusively from the connective-tissue cells of the theca interna, which liegin to enlarge even before the follicle finally bursts, and to charge themselves with lutein granules. Certain writers attribute the origin of these cells to the granulosa either wholly (Exner and Call) or in part ( Waldeyer) . Peculiar is Beulin's view in his Konigsberg dissertation, 18TT, that they are derived from the membrana propria folliculi. Benckiser's observations, 84. 1 , prove conclusively that in the pig the lutein-cells arise exclusively from the theca interna. This view I accept for man also, not only on accoimt of the accuracy of the observations made in support of it (see His, 65.2, and Frommann, 86.3), but also because specimens of my own show that there is no granulosa in the human corpus hemorrhagicum, while the young lutein-cells can be easily recognized in the fibrous tunica propria. In consequence of their site of development the lutein-cells and vessels form a band around the coagulum, and owing to its own growth this yellow band soon becomes folded. The central portion of the corpus luteum long remains distinguishable as a separate nucleus.

The exact history of the corpus luteum varies according as ovulation is followed by pregnancy or not. In the latter case the corpus is entirely resorbed in a few weeks ; in the former it persists until after the birth of the child. Wo distinguish accordingly the corpus luteum of menstruation from the corpus luteum of pregnancy, or corpus luteum verum of authors.

The corpus luteum of menstruation begins with a blood-clot. " The more recent the date of the menstrual flow, the fresher is the clot in the cavity of a ruptured Graafian follicle, and the less change has taken place in its surrounding wall. A few days later the wall begins to be enlarged and thickened, and this enlargement within a confined space causes it to become folded upon itself in short zigzag re<luplications, mainly at the deeper part of the follicle. As the process goes on the entire wall participates in tiie hypertroph}'. Its convolutions are extended and multiplied, often in a very complicated manner. They project into the cavity of the follicle, encroach upon the central (»lot, and become pressed against each other, forming by their coalescence a thickened, ghuidular-l(x>king envelope. Previously to the rupture of a Graafian follicle its wall is a uniformly smooth, vascular membrane, not more than one- fourth of a millimetre in thickness. After the rupture, its thickness increases to one-half a millimetre ; but as the foldings above described grow in number and in depth and crowd against each other laterally, the apparent thickness of the envelope thus formed becomes much greater, and may reach three or even four millimetres, especiall}' at the dt»epest part of the follicle. In this way there is pnxluced, during the intermenstrual period, a cot-pus Inteum^ occupying the substance of the ovary immediately beneath the superficial cicatrix which marks the site of the rui)tured follicle. At this time the central clot is red and gelatinous, while the convoluted wall is of a light rosy hue, mixed with more or less of a yellowish tint. Subsequently the whole structure diminishes in size, and the convoluted wall assumes a more decided yellow" (Dalton, 78.1, p. 18).

Leopold, 83. 1, distinguishes between the typical and atypical cor Eora, the former being those which start at the menstrual epoch and ave a blood-clot, a result probably of the ovarian hyperaemia, the latter beginning intermenstrually and having little or no blood. He says, /. c, p. 'M)i): **The t}'pical corpus luteum api)ears on the first day as a freshly ruptur«<l follicle, which has filled itself with bl(X)d; on the third day as an enormous blood-cavity ; al)Out the eighth day a thin cortex and a clearer nucleus are marked in the clot. From the twelfth day on, the cortex thickens and btx^omes foldetl ; by the sixteenth dny it becomes pale-red or yellowish. Toward the twentieth dav the nucleus shrinks marked Iv, the cortical band l)ecome8 moi*e and more yellow, and shoots in toward the centre in ravs and narrow folds, so as to leave by the twenty-fourth to thirty-fifth day only a small, pale nucleus enclosed in a much -convoluted brightyellow shell.

The corpus luteum of pregnancy begins in a similar manner to that of menstruation, but its growth continues. At the end of the first month its wall is convoluted, much thickened, and of a brilliant yellow color; the central clot is nearly or quite decolorized and constitutes a white or whitish firm central mass, which in nearly one case out of thrc^ has a central cavity with well-defined, sm(X)th walls. Sometimes a few fine bl(X)d-vessels |)enetrate through the lutein layer. The external convoluted wall continues to grow by encroaching upon the clot or white nucleus {corpus albicans)^ and at the same time the brilliancy of the yellow color diminishes. At term the white nucleus takes up alxmt one-third of the diameter of the corpus and is still distinctly (connected with the stigma, so that the lutein wall is interrupted at one point; the corpus as a whole is somewhat smaller than at from two to six months. After deliver\% resorption goes on rapidly (Dalton, 78. J ).

The brilliant yellow is especially characteristic of man ; in sheej) the pigment is pale brown, in the cow dark orange, in the mouse brick- red, in the rabbit and pig flesh-colored. Lutein is a crystalline Ixxly, soluble in alcohol, ether, chloroform, and benzol, but (jf its chemical nature we have no exact knowledge.

Physiology of Ovulation. — Concerning this subject and also concerning the functions of the corpoia lutea, we possess scarcely any knowledge. We have to consider only the relation of ovulation to menstruation and coitus.

Coste, 37.1, 454, 455, first showed that the discharge of the ova c<^incided with the period of heat in various animals. This was soon confirmed by Raciborski, 44. 1, and since then by numerous observers. Pouchet ('*Theorie positive," etc.) attempted to prove that this is also true of the human species, the menstrual period being taken, correctly, as the equivalent of the rut. In this attempt Pouchet has had many followers, especially among gjTisecologists. Coste, however, demonstrated long ago, 47.1, 22*2, that the bursting of the Graafian follicles may occur before or after menstruation, though it is most apt to occur during the menses. This conclusion of Coste's has been fully confirmed by Leopold, 83.1, w^ho made a very careful examination of twenty-five pairs of ovaries from women whose menstrual history was accuratelj' known.

It was Coste again, 47. 1, 183-185, who proved experimentally that coitus hastens in the rabbit the rupture of the Graafian follicles. Unfortunately he gives only two experiments, and since then they have not been repeated, so far as I am aware, either upon rabbits or other animals. But there are statements by many authors, Bary, Reichert, Hensen, 88.1, 58, Van Beneilen, 80.1, etc., to the effect that in the rabbit after coitus during heat the follicles are found to have burst during the tenth hour.

IV. Impregnation

Impregnation is the union of the male and female elements to form a single new cell, capable of initiating by its own division a rapid succession of generations of descendent cells. The new cell is called the impregnated or fertilized ovum. The production of cells from it is called its segmentation. For the theory of the relation of the elements to one another and to cells, see the following section.

In all multicellular animals, impregnation is effected by three successive steps : 1 , the bringing together of the male and female elements ; 2, the entrance of the spermatozoa into the ovum and formation of the male pronucleus ; 3, fusion of the pronuclei to form the segmentation nucleus. We proceed to consider these steps in their order.

1. The Bringing Together of the Sexual Elements. — This is effected in a great variety of ways, which, however, fall into two groups according as the impregnation is effected, a, outside the body of the mother; or, ft, inside. The simplest manner is the discharge of the male and female elements at the same time into the water, leaving their actual contact to chance, the method of the osseous fishes for the most part and of many invertebrates. An advance is the copulation of the Anura (frogs, etc.) ; the male embraces the female, and, as the latter discharges the ova, ejects the sperm upon them. In the higher vertebrates the seminal Huid is transferi-ed from the male to the female passages during coitus. The physiology' of this complicateil function does not fall within the scope of this work.

For a long time it was not known how the semen fertilized the ova; the problem was fruitful of fruitless speculation. The first step toward gaining actual knowledge was the discovery of the possibility of artificial fecundation by Jacobi in 1764. Spallanzani was the first to take advantage of this and to show that fecimdation implied a material contact of the semen with the ova, and thus to set aside De Graaf's notion of the "aura seminalis."

But not until fifty years later did the memorable experiments of Prevost and Dumas {Annales des Sciences Naturelles, 1824) establish the fact that the spermatozoa are the essential factors of fertilization. Again, a little over fifty years later, Hertwig and Fol showed that one spennatozoon suflfices to impregnate an ovum.

We have then to consider how the spermatozoon, after the semen has been transferred to the female, attains the ovum. They are found in mammals after copulation in the vagina and even in the uterus, but it is not clearly ascertained how they get beyond the vagina. It is probable that the}' travel through the female passages partly by the movements thereof, partly by their own locomotion, and enter the Fallopian tubes, though why or how is really unknown, and pass upward to meet the ovum. They are found in considerable numbers in the Fallopian tubes. The ovum meanwhile travels down the oviduct, it probably being impelled by peristiiltic movements of the duct.

The meeting-point or site of impregnation in placental mammals is about one-tliird, perhaps one-half, way down from the fimbria to the uterus. It is remarkably constant for each species. Nothing positive is known as to the site of impregnation in man ; but there is no reason to suj)pose, as is unfortunately often done, that the site is variable or different from that in other mammalia.

2. The Entrance of the Spermatozoon into the Ovum and Formation of the Male Pronucleus. — With our present knowledge, the assumption appears unavoidable that the ovxmi exerts a specific attraction upon spermatozoa of the same animal species. We observe, in fact, when artificial fec»undation is employed, that the sjiermatozoa swarm around the ova as if held by an irresistible impulse. This phenomenon occurs with every class of animals, even in mammals, whose freshlv removed ova were examined on a warm stage under the microscoj)o (Rein, 83. 1) . Stassano, 83. 1, has maintained that the eggs of echinodemis do exert such an attraction, and also a similar but less strong attraction upon the spermatozoa of allied species. But since the brothers Hertwig, 86. 1, have found by their experiments with sea-urchins that hybrid impregnation takes place more readily after the ova have l)een kept awhile, Stassano's view involves the further assumption that the specific nature of the attraction fiides away during a few hours. Verv suggestive in this connection is Pfeffer's (^^Untersuch. Bot. Inst." Tubingen, Bd. I., Hft. 3, 1884) discovery that certain chemical substances may attract moving spores, etc., to definite six)ts. It is conceivable that the ovum may draw the spemiatozoa toward itself by chemical influence, acting as an attracting stimulus.

There may be mechanical devices to facilitate the entrance of the spermatozoon ; this is, perhaps, generally true of all ova with micropyles servinff for the ]>n8»age of tho Bix?rmntozoa. A rarefu] study of such (tevices m the fockrojicli has l)ecii made by J. Dewitz, 85, 1, who found that thp mutionu of the Bix>rmatozoi* of this insect are peculiar and adapted b» iiii'reii«e the pR>l)iibilitj- of their passing thnnigh one of the nii(Toi)yle8 of the ovum. In ova without micropyles, among whii-li those of niamrajils are included, the spermatozoa may, so far as we knoiv, penetrate any part of the envelopes.

In the nilibit (Rein, 83.1), alxiut ten hnure after coitus, the ovum is found nearly half-waj- throiit^li the oviduct and surrounded by many siiemiat<>zoa — perliaps a hundred, more or U>ss. These are all, <jr nearly all, in active motion, for the most jKirt pressing their hetuls against the zona ntthata. Sevend of them make their way through into the interior of theovum. According to Hensen, 76,1, only those si)ennatozoa which enter the zona along radial lines can make their ^ ,- , way through: those which take oblitjue v_ -—- '^'>/' / courses remain caught in the zona. Fig. 4(1, "^. ^ - -" / / and mav still bo sein there during scgmenta- " " —-^"^ tion. As the ovum at this time is already ^^^^^ [n;m'^'iIi""miJtHe^f'uJi fully mature<l, there is a siuice between the oviciuit atmu piKiiu^n bmire contracted yolksand tiiezona. Inthisspace, nwvipli^ u'^in-tu^^onii^i?"^ as well as ill the zona itsi'lf, several siR-rma- £;,'!"n™,I"«i;.™Bi?,'^aSbMi; tozoa may lie observed at scattered jxiints. tn and wiciiln tiio »««. Aftrr The female pioniK-leus is pres<'nt. having ^"^^'■ been rtr-fonned since the exjailsion of the si-cond jwilar globule from the ()vum while in the ovary. One s[>ermatozoi m gets into the yolk proiN-r, and its entrance api>iirently prevents the [lenetration of other s])ennato2<>a — how is nndetennine<l. The tail of the 8i>ennatozoon soon disapiwars, while the head enlarges, pi-olmbly by the imbibition of ritiid from the surrounding yolk, and thus Ixvomes a nucleus-like Uxly — iUe iiiah' jirininrlcus.


passage of the sjwrmatozoa throngb the zona wjis fii-st discovered by Martin Barry in is4:i. and although his statement was receiveil with considerable hesitatiim by his contemporaries, it has since had comi>etent contirmation i-ojieateillv. Wanieck (Bull. Soc, Xati.. M(K«K.u. XXIII.. '■»>) is s;ud to have Ix-en the first (ISSO) to si'e the two pronuclei, but their significance was not i)erceived. The nature of the male immucleus was first recognizi?d by Oskar Hertwig. who traced its genesis in the ova oi ecbinodenns from the apermatozfion. The fact that the male pronucleus is the metamorphosed spermatozoon has since Ik-cu continued bv Selenka (" Zool. Stud.," I.I: Ed. van Benetien. 83.1; Sussl)aum, 84.2; Eberth, 84.1; Platncr. 86.1, and others.

Although a numlKT of the spermatozoa make their way into the perivitelline space, probably always one alone normally enters the yolk to there form a ])roiuicleus. The best observers are agreed upon this point, and in all s]>ecies the obser^-ations upon which have covered the whole series of steps in the impregnation, there has been foimd in norma! cases always a single male jiromieleus. Schneider's statements to th<? contrarj- have l>een definitely correcte<l. Bamlieke, 76.2, C. Kupffer, 82.1, and Kupffer und Benecke (" Befruchtung Neimaiige," 1878), have observed that several spermatozoa actually enter the yolk in batrachians and Petromyzon. Hertwig, however, found only one male pronucleus in the frog, and there has as yet been no evidence adduced that several spermatozoa are concerned in the final phaaes of impregnation. Fo! observed that star-fish e^s are normally impregnated by one spermatozoon; but if they are exposed to the action of carbonic acid they may, while so poisoned, bo impregnated by several spermatozoa, and the subsequent development in this ca.se is abnormal : apparently each pronucleus becomes a separate centre of development.

The manner in which additional spermatozoa are excluded after the first has entered is still under discussion. In cases where there is a single micropyle, which is used for entry, it is possible that a portion of the first spermatozoon may remain to close the passage, or that in going through it sets in action some mechanism by which the opening is automatically shut. Where there are several or many micropyles, as in some insects, or where the en%'elopes may be pierced at anj- point, as in mammals, there must be some other device. Fol has maintained that this is found in the star-fish in the rapid formation of a membrane around the yolk immediately after the entrance of the first spermatozoon ; but Hertwig affirms that this membrane pre-exists. Selenka (Biolog. Centralbl., v., 8) describes the fertilization of the ovum of a nemertean worm; several spermatozoa enter within the vitelline membrane; the yolk contracts slowly. After a time the two polar globules are expelled, and before they separate from the yolk one spermatozoon passes into the yolk between them ; the globules then break oflE and are knocked about by the si^rmatozoa in the , peri vitelline space. In this case there seems to be a portal opened just long enough for one spermatozoon to enter. As the phe- - - — nomenon to be explained is com BtfHHi* wlth^a^ spermnt-iiooir "iJl"".^nUTrnK* tW "1<"1 *•> ^^^ OVa, itS CaUSation is

mforopyiB, mi,- j, v. i-rivin-iiin,- Bpa^-r; i. iodb uresumabh' fundameiitallyidentk. joik. aftW caiberitt tical m all cases. • Beyond this

siinnise our present knowledge does not permit us to go. The hypothesis may be siiggested that the attractive power of the ovum is annulled or weakened by the formation of the male pronucleus. This hj-pothesis was first suggested by Minot (Buck's "Hdbk.," IV., (i), and has since been elalx>rated by Whitman, 87.3, 239-243.

It is probable that the tail of the spermatozoon, when that appendage exists, disappears within the yolk. In a land-snail, Arion, Platner, 86.1, has traced this process very clearly. Only a portion of the tail enters the j'olk, but the part within acquires the property of staining readily, and so may easily be observed. He reports that the head and tail separate; only the head conjugates with the female pronucleus, while the tail still remains distinct even after segmentation has been initiated, Fig. 44. The disappearance of the tail has been recorded by most observers. As Hertwigsays {loc, cif., p. 23), all these careful observations yield the assured conclusion that the head of the spermatozoon, and the head only, becomes the male pronucleus.

While the spermatozoon is passing through the ovic envelopes, active changes occur in the yolk. Of these the most constant, as well as the most obvious, is the formation of a slight protuberance on the surface of the yolk, rising up toward the sjjermatozoon. This protuberance may remain, as in echinoderms, until the spermatozoon meets it and by penetrating it enters the o^^^n, or it may retract before the spermatozoon passes through the envelopes, and even withdraw, as in Petromyzon, Fig. 41, so far from the advancing spermatozoon as to form into a depression on its own surface, Fig. 41. The protuberance lasts only a few moments. In Bufo, according to Kupffer, several spermatozoa enter the yolk and a protuberance rises toward each one, as if the yolk were actively striving to reach the male element. The protuberance always consists of fine granular protoplasm, which contains no deutoplasm, and is closely connected with the nucleus. The size of the protuberance is variable. In Petromyzon there is a large hummock of protoplasm, which contains the nucleus and in which both pronuclei form and unite; during these processes the protoplasm of the hummock is separated from that of the rest of the ovum by a special membrane, which disappears immediately after the pronuclear copulation. While the two pronuclei are meeting the hummock flattens out and the protoplasm forming it travels centripetally together with the pronuclei (Boehm, 88. 1 , r)50, <J51 ). Whether the hummock in Petromyzon is homologous with the much smaller protuberance in other ova I am unable to say.

The relative size of the two pronuclei varies considerably in different species, and is probably a secondary and unimportant relation. Each pronucleus when it first appears is small and gradually enlarges, apparently by the imbibition of fluids from the surrounding yolk. Now the time when the spermatozoon enters the yolk may be either after or at some stage during maturation of the ovum. If it enters earh', as in Limax (Mark), the male pronucleus enlarjj^es e(|ually with the female. Fig. 4"^; but if late, as in the allied Arion (Platner), then it appears. Fig. 44, considerably smaller than the already swollen female pronucleus. O. Hertwig, in his third pajier on maturation, p. 171, first gave this explanation and pointed out that in the star-fish ( Aster ias), if the impregnation is prompt, the male i>roniicleus becomes as large as the female, but if impregnation is delayed for four hours the male pronucleus remains mucli the smaller of the two. Again in Hirudinea, Fig. 42, many Mollusca, Nematoidea, etc., impregnation usually takes place l)efore the formation of the polar globules is completed, and the male pronucleus is accordingly as large as the female. In Echinus, on the other hand, where the polar globules are formed in the ovary, the male pronucleus is always small.

Concerning the path of the male pronucleus we |>ossess little information. O. Hertwig and Bambeke have found that in certain amphibiaii ova the spermatozoon leaves a trail (Pigiiient-Strasse) apparently by carrying along with itself some of the pigment grauules from the surface of the ovum. Roux, 87. 1, has studied this path in the frog's ovum and finds that it consists of one limb, the line of penetration through the cortical layer of the ovum, which is nearly perpendicular to the ovic surface, and a second limb usually forming an angle with the first and leading directly to the female pronucleus. 1 U j The force which draws the pronuclei to ^ gether is unknown. We can only say that,

'^ is Whitman has thoughtfully expounded, -,,,^^ 87-3, there is a relation between the nu Fio 4r"iI;rT\efheiiti thiw cl^us and the protoplasm of the ovum, such houra fttier layini niaia / that the nucleus tends to fake a central posiK^obuW "AtieVo tfrrtwiK*" " tion. When the polar globules are formed the nucleus becomes repellant and drives itself centrifugall> but the protoplasmic attraction remains and draws in the spermatozoon. Subsequently both pronuclei are attractetl towanl the centre and toward each other, and the curved routes the pronuclei otten take are the resultants from these two attractions

3 Fusion of the Pronuclei.— Each pronucleus is usually found surrounded b> n i-pace a little clearer than the rest of the yolk. Usually the yolk around this clear space presents a radiating appearance, which is knpwii as the aster. Fig. 4;S ; but this appearance is not con ■ Btant, nor is it known how it is caused. Frommann, 89. 1, 3'Jo, states that in the egg of Toxojineustes the astral rays are formeil by very irregular rows of angular granules, which may lie separately, or bo strung together by tine threads, or like a row of pearls, and are irregularly connected by cross-tlireads. The great regularity usually pictured is purely diagrammaiic. As the granules described by Frommann ai-e part of the reticulum of the ovimi, we may say that the astral figiu-e results from the arrangement of the protoplasm. Mark, 81.1, was unable to see it in Limax, and Rein, 83.1, aiuldnot detect it in the rabbit. In Arion,'as also in Peti-omyzon, according to Boehm, 88. 1 , apparently only the male pronucleus has an aster. Fig. 44. At one time it was assumed that the pr<muclei acted aa centres of attraction uiion the yolk, and that the asters were (lue to their direct influence; but since, as in Arion, Fig. 44, the pronucleus may move away while the aster remains behind, it follows that tlio relations are more complex than this assumption indicates, since the aster exhibits a certain independence of the jironucleus. This is confirmed by Flemming's olwer^'ations iJoi: c;/., p. Ill), that when the asters first apjiear in echimxlerms, the centre of radiation is not the pronucleus itself, but a clear space just alongside. Frommfinn, 88. 1, VM. modifies this statement by recoi-ding that the jxjsition of the centre of the male aster varies in Toxopneustes and may l>e »t one side or the other of the male pronucleus or coincident with it. Boehm, 88.1, ii50,Taf. XXV.. Fig. ;to</. notes thesameiwuliarity in the eggs of Petromyzon.' These statements recall the fact that the asters in indirect cell-division sometimes radiate from a clear spot at the tip of the spindle. Some writers have considered the aster an expression of magnetic force within the ovum — a fanciful notion without any evidence to supi>ort it.

In the rabbit, Rein, 83.1, both pronuclei lie at first eccentrically, but they move toward each other and toward the centre, meeting, however, before the central position is attained. As they near one another, both pronuclei perform active amoeboid movements ; after they meet they still continue their amoeboid movements and move together to the centre of the ovum ; one of the pronuclei assumes a crescent shape and embraces the other, Fig. 45. At this time the yolk displays a radiate arrangement ; from analogy with other animals it must be assumed that the two pronuclei fuse into a single nucleus, which is therefore an hermaphrodite structure, and which, after a certain pericxl of repose, itself divides and so begins the cleavage of the yolk.

The place where the pronuclei meet varies. Apparently the female pronucleus of itself moves to the centre or near the centre of the ovum ; also the male pronucleus approaches the female as speedily as possible. If now impregnation occurs early, the two pronuclei meet peripherally ; if late, they meet near the centre. In the former case they move together, as in the rabbit (Rein), to a central position. The observations so far made indicate that after they meet the pronuclei both perform active ama'boid movements, whicli continue for several minutes. Selenka maintains that the female proimcleus sends out processes which embrace the male pronucleus, but this has not been confirmed. Finally, the two pronuclei unite, but the process of union is very obscure, never having been satisfactorily ol)serve<l. Apparently the membranes of the pronuclei, where the two are in contact, are dissoh^ed awav and the contents mix. The Ijest account known to me of the fusion of the pronuclei is that given by Boehm in his memoir, 88.1, on Petromyzon. The outline of the female pronucleus is still diffuse a quarter of an hour «fter fertilization. The head of the spermatozoon (male j)ronudeus) breaks up into four, more rarely five, granules. The female ])ronucleus moves centri[x?tally, and ac<iuires a distinct membrane. The j)ronuclei meet, the male granules liaving meanwhile multiplieil by division. Al)out this time the female pronucleus also breaks up into granules. We then havo a clear spot which is the centre of an astral radiation, nc^Kt this a bunch of male granules (Boehm's Spermatomeriit'll), and next that a bunch of female granules (Boehm's Ovomerifen)^ the whole making an elongated IkkIv lying at right angles to the radius of the ovum. Three hours after fertilization the two bunches are fused together and are no longer distinguishable. Each '* Merit" consists of a body containing one or two chromatin specks [Mtcrosomen). In the Crustacea, according to Weismann and Iscliikawa, 88.4, the two pronuclei, when they meet, resemble ordinary membranate nuclei ; where they come in contact with one another the membranes dissolve away and the contents of the pronuclei mii^le. In Ascaris the process is more complicated. We may say, therefore, that the fusion of the prt much i <* ibe *?s eiittal phfiiomenon, and the method of the fit.'iioii is secondary in importance.

Fig. 48.— Ovum of Sa^itta with two proniiclfi. Aft«^r O. Hertwijf. Ai'ouiul each proDUcleus is shown the ustcr.

. anduu AH d Afler Plainer Tl?

D u ro. aDd mulualli on pn-sBPil rliu two larRB 'KaryoHomrD id it an- d ]1 remainn, ai. B iiiioiiii the comnieri,iiiB le. la bntK ura [he lall a; u( Ihu eperu


Another point deser^-ing mention is the rotation of the copulated nuclei. See Frommann's article on "Befruchtung" in "Eulenberg Cyclopaedia," p. 56f.

Now since the head of the spermatozoon is developed chiefly out of the chromatin of the nucleus of a spermatoblast, it follows that impregnation is essentially the addition of chromatin to the nucleus (female pronucleus) of the mature ovum. After the union of the pronuclei follows a period of repose, during which the yolk enlarges until it again fills or nearly fills the space - within the 20H« radtaia ; a little room is

left, which is chiefly occupied by the polar globules. The significance of the contraction of the mature and the exjumsion of the impregnated yolk is unknown. In <'ei-taiu casf 3 the parts of the segmentation '. ' ; ' nucleus which are derived from the nisde

f ' / pronucleus remain distinpuiahablo. This

V ,- . / is notably, according to Plainer, the case

' : / with Arion. The segmentation nucleus

^ \ „-^ contains a numl)erof nucleolus -like Ixxlies

Fig. «.-ovum ..f n mbhii nev- (KaryosomeD of Plainer, Fig. 44, A. w), S!^«i^a7-Sfri«^'K.';-^ with a distinct round outline, and a few Ki« »"n^' " """" "'"*^" granules of chromatin. These lK>dies are " " of two kinds. Fig. 44 ; the smaller and

more numerous are produceti by the female pronucleus, while the two lai^r ones arise from the division of the head of the spermatozoon.

In the later stage, when the nucleus is changing into the fii'st segmentation spindle, Fig. 44, B, the two large male •* Karyosomen" are still distinct, and have each their chromatin gathereil in little particles around the periphery. Edouard van Beneden, 83.1, goes even further, stating that in Ascaris the chromatin from the two pronuclei can l)e distinguished in the nuclei of segmentation, and that when it divides, both the male and female chromatin loops divide also, so that the resulting nuclei are truly hermaphroditic.

V. Theory of Sex

Sex is a term employed in two meanings, which are often confused but which it is indispensable to distinguish accurately. Originally sex was applied to the organism as a whole in recognition of the differentiation of the reproductive functions. Secondarily, sex, together with the adjectives male and female, has been applied to the essential reproductive elements, spermatozoon and ovum, which it is the function of the respective sexual organisms (or organs) to produce. According to a strict biological definition, se.vuality is i;he characteristic of the male and female reproductive elements, and sex of the individuals, in which tho.se elements arise. A man has sex, a spennatozoon sexuality. Sexuality is primitive and essential, and sex is dependent ujxm it. We have to consider, Ist, the nature of sexuality-; 2d, the origin of sexuality; 3d, the nature of sex.

Nature of Sexuality. — The essential fiic'ts of sexual reproduction are: That two bodies, partaking more or less plainly of the character of cells, fuse together into a single lx>dy, which seems in every known resj)ect to be homologous with a uninucleate cell, and which imdergoes a series of divisions (segmentation of the ovum) resulting in th(* production of an increasing number of new cells. In all the higher animals (and plants) the two bodies are obviously different. In all metazoa one Ijody, the ovum, contains a store of nutritive material and has a sptH'ial enveloj^e of its own ; the other, the spermatozoon, is small and provided with means of active locomotion; the details of their fusion, which is known as the fertilizatiim or impregnation of the ovum, have been descrilietl.

The only hyix)thesis, as to the nature and mutual relations of the ovum and spennatozoon which rests, such is still my opinion, on much bjLsis of fact is Minot's "Theor}' of the Genoblasts," 17.47. This hypothesis is l>ased upon three categories of facts: 1st, Sexual reproduction is effected by the union of a male and female element, which proiluces a cell; this cell is, therefore, hermaphroditic, or, perhaps, one should sjiy, asexual or neuter, since it is neither male nor female, 'id. When the cell, which gives rise to the female element matures into an ovum, it undergoes a remarkable process of umHjual division, known as the extrusion of the polar globules. In other wonls, the cell divides into three Ixxlies — a, two polar globules; /;, a single female element. In some cases the polar globules subdivide further. M. When a cell divides into the male elements there remains one cell which does not form a spermatozoon. In mammals it is probable that the parent-cell divides into three cells, one of which, h, remains to form the bas«.» of a Sertoli's column, and two of which, a, subdivide further to produce the spermatoblasts and ultimately the spennatozoa. Unfortunately our knowledge of the development of the spermatozoa is extremely unsatisfactory', no two authors agreeing, so that extreme caution is necessary. There are, however, reasons for thinking that the statements just made in regard to mammalian spermatogenesis correctly specify the essential steps, and it is probable that the essential steps are the same throughout the animal kingdom. Assuming then that the view of spermatogenesis here adopted is correct, our further deductions from the * premises are almost self-evident. In the cells proper both sexes are potentially present; to produce sexual elements the cell divides into its sexual parts, the genoblasts ; in the case of the egg-cell the male polar-globules are cast off, leaving the female ovum (oospore of Balfour) ; in the case of the sperm-cell the male si>ermatoblast8, which by the hj'pothesis of Minot are homologous with the polar globules, multiply considerably, and their descendants give rise by further specialization (in mammals of their nuclei) to the male elements, while the parent-cell, or homologue of the oospore, atrophies. In both cases the sexual cell separates into a single female element or ihelyblast, and probably two male elements or arsenohlastSy which are capable of multiplication by division; but in one case it is the thelyblast, in the other the arsenoblast, which is preserved, differentiated further, and utilized. To make a complete cell there must be a union of the male and female, and this is accomplished by " impregnation of the ovum/'

Minot's h\TX)thesis is strictly morphological and offers us no insight at present into the physiological aspects of sexuality. It has been adopted by Balfour, and Ed. van Beneden, neither of whom cite Minot. Since the theory of the genoblasts was first advanced in 1877, it has been confirmed by important discoveries, especially by the series of investigations which have proven that polar globules, as stated in the section on the maturation of the o\^m, occur in all classes of the animal kingdom, and, secondly, through the investigations on the relation of the polar globules to parthenogenesis. The general, may we not say the universal, occurrence of the formation of the polar globules as a necessary step in rendering the ovum capable of impregnation, is, of course, a very important confirmation of the theory, since the theory assumes that the production of the polar globules is the essential step in converting the egg-cell into an oospore or thelyblast.

Minot, in his original article, briefly indicated the application of his theory to parthenogenesis, and the question was more and ably discussed by Balfour in his '* Comparative Embryolog>%" I., 63--G4. In his article of 1883, 47, 367, Minot is more explicit. He says : " If one assumes that the ovum becomes female by the removal of the polar globules, then it must remain asexual so long as no globules are formed. If one further assumes that no polar globules are formed in ova, which develop parthenogenetically, then these ova would remain simple cells, and their reproductive process would depend on ordinary cell-division. If the glolniles are develope<l then impregnation is an unavoidable preliminarj' of further development." In other words, parthenogenesis is only an extreme case of asexual reproduction and in nowise the development of a female element (oospore or thelyblast) without impregnation. The correctness of this view has since become extremely probable through the observations of Blochmann, 87. 1, 88. 1, of Weismann, and of Weismann and Ischikawa, 88.2, 4, who find that in parthenogenetic ova there is only one polar globule formed, while in eggs retjuirinjr fertilization there are two. Now by Minot's theory the cells must be hermaphroditic in order to develop, and the egg-cell becomes a thelyblast by the ejection of two polar globules; if, therefore, one polar globule is removed and the other not, the egg-cell retains part of its male constituent. The significance of the tiro polar globules has already been discussed, p. 05; Weismann's interpretation is considered in the following section on Heredity.

To my theory uf the genoblasts, I feel justified in making an essential addition — namely, that sexuality is a relationoi substances or forces, and not dependent on any special substance. The chief evidence in favor of this assumption is the fact that in all male elements the proportion of protoplasm to the nucleus is small, while in female elements (thelyblasts) it is small; and, moreover, to produce spermatozoa there is an excessive growth of the nuclei, while to produce ova there is an excessive growth of the protoplasm. It is remarkable, as Minot has demonstrated (Address, Proc. A. A. A. S., is'JO), that a relative increase of protoplasm is the anatomical characteristic of senescence. The ovum resembles an old cell, the spermatozoon a young cell, and these resemblances cmmot be considered fortuitous.

There is no material basis of sexuality in the sense that there is any visible male or female substance known to the biologist, nor is it probable that a male or female substance exists. The functions of life, according to our present conceptions, are not each connected with particular chemical comjx)unds or with particular visible constituents of the cells, but rather depend upon the complex interrelations of numerous different substances, which enter into the composition of the tell. There are certain functions which are connected more intimately with one part than with another — as, for instance, contractility with the protoplasm, heredity with the nucleus; but even in these cases we cannot say that the functions in question could go on without the interplay of the other portions of the cells. The genoblasts contain nuclear substance, protoplasm, and enchylema, and we can ascertain the sex of a genoblast only by observing its historj% not by any direct test. It is probable that male or female sexuality is an intracellular relation of parts, some modification of the interplay of forces within the cells, and for the present this view must hold against the opix)site view that there is a male matter and a female matter.

Several interpretations of the polar globules have been advocated, which are incompatible with Minot's theory. The first of these is that of Whitman,* who, in his first article on the development of Clepsine, 78.2, p. 4S, maintains that a series of cell generations is produced by a series of divisions, and the separation of the polar

  • Ci>mpaiv also U. Hertwig. 90, 1.

globules is merely the last of these divisions. Inasmuch as this view overlooks the fact that polar globules are part of the process of maturation and that no oviun can be impregnated until they aie formed, and the further fact that the products of division (globules and oospore) are extremely unlike, while in ordinary divisions the two daughter-cells have close resemblance to one another — inasmuch as these fundamental facts are overlooked, it seems to me that Whitman's explanation cannot be adopted.

Allied to Whitman's view is that of Butschli, 84. 1, who, starting from the idea of a sexual colony, such as is found in certain unicellular animals (Flagellata), considers that the tendency to form such colonies is preserved in the metazoa, and shows itself in the bundle of spermatoblasts and in a more rudimentary form in the egg-cell, forming a colony with its two polar globules. The essential objection to this view is that it overlooks the fact that the divisions of cells to produce the sexual products are divisions into unlike bodies, while in the sexual colonies of the Flagellata the divisions are, so far as known at present, into like cells.

O. Hertwig's criticisms, 90.1, against Minot are based on the study of the differentiation of the sexual elements of Ascaris. He overlooks the fact that the theory of Minot depends on the origin of the sexual elements, not on their differentiation; yet nothing is known as to the origin of the genoblasts in Ascaris.

Besides the theory of sex already discussed, there are three others which must be noticed The first of these has been advanced by Sabatier, and defended by him in a series of articles, several of which have been reprinted, making in their reprinted form the fifth volume of Sabatier's ** Travaux. * Sabatier considers that the cells are neuter or hermaphroditic, agreeing in this respect with Minot, and that the casting off of the male portion converts the cell into a female element, and vice versa^ but ho goes farther than Minot in attempting to specify which, parts of the cell are male and which are female. He directs attention first to the fact that in certain invertebrates there is a central mass (Bloomfield's blastophore)^ to which there are attached spermatoblasts or si)ermatozoa. He endeavors to prove that this is the primitive method of spermatogenesis, and concludes that the male element is j^)eripheral, and the product of a centrifugal action. He directs attention, second, to the various products that are thrown off from the cell, which ultimately forms the ovum. Summarizing his conclusions in regard to the e^^ {/. c. V., •202-203) he says: **lf we recapitulate now the various groups of globules which are eliminated from the ovule, commencing at the asexual cell-stage of its life up to the moment when it attains the complete dignity and signification of an ey^^^ we see that there may be:

    • 1st. Globuleti precoce.s on du debut ^ which tecome usually the elements of the follicle and give, so to speak, the first impulse to the march of the cell toward sexualitv.

" 2d. Globules tardifs, which are at times formed well tefore the epoch of maturity, but are eliminated at a late }^riod, and sometimes very near the maturity. They are all formed, as are the globules

  • "Travaux ilu Lttl»oratoired<» Zoolos:i»> de laFa(*ult<^ des Scienci's de Montpellier et de la Station ZooloKique de Cette." Irt* Ser., 5me vol

precoces^ by simple differentiation in the miilst uf the i)rotopla8m and without karyokinetic phenomena.

•' 3d. Globules, which are contemporary with the period of complete maturit}', and of which the elimination accentuates in the egg a very pronounced attraction for a male element coming from another cell, or even from another organism. These are the globules de maturation parfaite. Most of these globules result from phenomena of cellular division, and form the polar globules properly so-called."

From this quotation it will be clear that Sabatier classes together the follicular cells surrounding the ovum, the non-cellular masses excreted from the egg-cell during its development, and the polar globules. All of these are — so he maintains — thrown off from the central ovum, hence he concludes that the female element is central and the product of a centripetal action. In brief the male element represents a centrifugal force, the female element a centripetal force.

A. Prenant has adopteil a theory which is apparently a modification of Sabatier's, but until his memoir is published {Journ. de VAnat, et Physiol., 1802) discussion of his theory must be deferred.

I am unable to accept Sabatier's theory for many reasons, of which the following may l)e mentioned : 1st. It cannot be shown that the differentiation of the sj^ermatozoa does occur typically at the periphery ; on the contrary, in the great majority of cases, it is distinctly polar, since it takes plac« at the inner end of an epithelial cell. 2d. It is impossible to maintain a homology- l)etween cells and masses which are not nucleated at any period of their histor}', and Sabatier's views as to the maturation of the ovum oblige us to draw such an homolog}'. 3d. Sal)atier, to establish the centrifugal removals, which produce the ovum, relies largely upon the history of the globules tardifs, which, therefore, must by his hypothesis bo male. He bases his defence largely on observations on the " testa-cells " of Ascidians, which he considers to belong under the head of globnles tardifs; but these observations have been calleil in question by Fol (Kecueil Suises Zool., Xo. 1),* so that there is doubt as to one of Sabatier's chief foundations. Xow some of these globules — supposed to be male — contain no nuclear substance, yet all the sexual elements, which we know positively to be such, do contain nuclear substance.

Balbiani's theory, 79. 1, is the exact inverse of the two previously mentioned ; for him every sexual element is the product of the copulation of two elements: 1st, the epithelial cells of the follicle, which are male; 2d, the I'rei, which is always female. Balbiani has not observed any such copulation, nor has he any valid indirect evidence of it to bring forward; on the contrary, he disregards in several i*espects what others consider elementary principles of histology'.

Nussbaum's theory appears to me valuable and suggestive. It was first advance<l, so far as I know, in 1880, though similar conceptions are to be found in earlier writers. Nussbaum, 80.1, starts with the conjugation of two similar unicellular individuals, as occurs in certain protozoa ; the two individuals fuse, and after fusion

  • For Sabatier's answer we same Recueil. No. 3. fi

divide into successive generations of cells. He next points out that in the higher animals all the sexual differences are secondary not only in the so-called " secondary sexual characteristics, " but also in the sexual organs themselves. He then goes on to emphasize the

Presence of the sexual cells (Ureier of German authors, Hamann's Jrkeimzellen) ixi the embryo, and maintains that as these both give rise to the sexual products the ovum and spermatozoon are strictly homologous cells. He writes, p. lOG : There come together during impregnation accordingly not two heterogeneous elements which complement one another and together form a whole, but rather there come together two homt^logous cells, of which one to facilitate conjugation is transformed into a more movable body ; the other is laden with nutritive material, and is furnished with protective devices." And again, p. 113: " The differentiation of sex is not the transmission of two originally united functions to the differing descendants of a common original Anhige; it is rather the variation of homologous cells for the better achievement of their conjugation." The sexual elements, according to Nussbaum, are cells which are set apjirt for reproductive functions from the rest of the cells of the body, and there is no primary difference between male and female. He does not consider in any way the significance of the jwlar globules or Sertoli's columns, and therefore does not argue directly against Minot's theory. His generalization that separate cells alike in character are set apart early in embryonic development to form both the male and female elements is a very important one, and has been adopted by embryologists. Weismann accepts it and applies it to his thcijrv of hereditv, and it has received a valuable confirmation in Hamann's pajx^r, 87. 1. But tliis pconeralization leaves the question of the final differentiation of the Ureier into sexual elements imtouched, and is not necessarily in any way in conflict with the conception of that differentiation advocated by ilinot.

It seems to me, therefore, that although Minot's hyi)othesis cannot be proven at jjresent, yet there is no other hyix)thesis of sex having nearly as strong evidence in its favor.

Origin and Objects of Sexuality. — The origin of sexuality is involved in much ol)scurity. In the lowest unicellular organisms there is certainly no clear sexual differentiation, and some biologists assert that there is nothing comparable to sexual reproduction, but the observations are far too imj)erfect at present to justify any such assertion. The question involved is, whether sexuality is coextensive with life or not; in the latter case it is the result of evolution from asexual organisms, and is a secondary and not a primary or essential characteristic of life. The problem is, therefore, a fundamental one, but we cannot hope for its solution until our knowledge of the lowest organisms is greatly extended.

The precursor of the sexual process is undoubteilly to be found in the conjugjition of two similar cells, which fuse into a single organism, as occurs in (»ertain cryptogamous plants and among the protozoa, notably the rhizopcwls. In the next stage the cells which fuse together are obviously different, as in the Flagellata. If now we pass to the colonies of the Flagellata we find that certain cells only act as conjugators, and thus we appn^ich the disposition of the multicelliilar animals, Metazoa, which have bodies composed of cells, certain of which produce the sexual elements, and these elements conjugate. In conjugation and impregnation alike the process is the fusion of a nucleated protoplasm with another nucleatetl protoplasm of different origin. In plants also, as we ascend from the lower to the higher forms, we find the differences between the conjugating lK>dies to incrciise: thus in zygophytes the conjugating cells are alike, in phanerograms the ix)llen and ovicell are unlike. The question arises whether the conjugation of the like or of the unlike protoplasms (or, in other words, of similar cells, or of genoblasts) giveg the clew to what is essential. Is the dissimilarity of the conjugating bixlies essential? If Minot's theory of the genoblasts is correct it is probable that the dissimilarity is essential, in which case it is conceivable that when similar cells conjugate each cell contains both male and female, and the male of one saturates the female of the other, and rice rersa. On the other hand, the whole tendency of evolution is from the simpler to the complex, and, a priori, it is more plausible to consider that complete sexualit}' is a differentiation of a simpler process rather than the mere separation of what was united in one cell. The last-mentioned conception is undoubtedly tlh.» one which would appeal to most biologists at the present time. Yet we see that the functions which exist in a cell do undergo separation, so that they become excessively predominant in certain cells ; for instance, the nervous functions have been thus selected out for the sui>erfluous endowment of certain cells, and it appears to me perfectly conceivable that male and female may Ix? united in a unicellular organism just as completely as assimilative and nervous functions, and as these latter are differentiated, so, too, are the former.

The above considerations, and others which might be given, were it worth while to lengthen the discussion of so obscure a subject, lejvd me to the hypothesis that aexualittj is coexteiiHive trith life: that in protozoa * the male and female are united in each of the conjnqatimj cells, and iniprerf nation is double; and, finalltj, thai in the nietazoa the male and female of the cells separate to form (jenoblasts or true sexual elements, and impregnation is single. It need hardly be pointed out that this hypothesis is purely tentative, and may have to be I'ejected altogether when we have sufficient knowledge to decide as to its validity.

The object of sexuality is, likewise, known only by hypothesis. Three views are to be considered : its puqKJse is, 1st, rejuvenation; 2d, to produce variability; 3d, to check variability. 1st. The theory that the purpose of sex is to proiluce a young organism is very old, and is based on everN'-day observation ; it involves, as its corollary, thait organisms l)ecome old, and thereby incapable of maintaining their own existence. That sexual re])roduction d(X^s produce a young organism is the universal law ; it is also true that ever^'- young organism does possess certain morphological and physiological characteristics by which it may be distinguished from an old organism. "When sexual repnxluction occurs life proceeds in cycles; the sexual conjugation produces a single cell, which divides again and again, until at last the process cannot proceed further ; tiien a renewed conjugation follows and a new cycle of cellgenerations ensues; in the higher animals the cells remain together as they multiply ; in the protozoa the cells each lead a separate life, but in both the cell-cycle is dominant ; the body of a metazoon is comparable to the set of individual unicellular protozoa resulting from one sexual act. In one case the cells of a cycle remain together, in the other they separate. So far, then, as it is known to occur, the sexual process is a rejuvenating one ; but this does not prove that all living organisms reijuire sexual rejuvenation from time to time, nor does it prove that there is no other means of rejuvenation. It may be that all cells as they divide asexually lose their growth-power, so that there comes a time when there must be a rejuvenation or restoration of the growth-power, but it is improbable that sexual reproduction is the only means to effec*t the necessary restoration of vitality. 2d. That the object of sex is to increase variability and so atford a wider scope for natural selection has been maintained by Weismann. At first sight the notion of the mingling of tw^o hereditary strains of different character producing variety in the offspring seems very plausible; but the notion does not l)ear examination, for it renders the commencement of variability impossible, and fails to account for the divergence in the offspring of the same parents. 3d. The view that sexual reproduction checks variability has been advanced by Hatschek, 87.1, '380, who points out that the mingling of hereditary strains tends to restore the si>ecific norm, since in the long run the variations counterbalance one another. Gallon has shown that in human stature the tendencv of hereditv is to restore the normal height, and the same is presumably true of other characteristics. I am strongly inclined to accept Hatschek 's theory, and to maintain with him that one result of sexual reproduction is to correct variations and so preserve the specific type.

  • Very possibly this Im uot true fur all i»rotozoa for then*, may Ix? pn.»ti>z<>a with true jjeuo

Nature of Sex. — Sex, as we encounter it in the human species, is the result of a long evolution affecting a large numl)er of organs — I>erhaps all of the organs — s<^> as to result in characteristic differences between the male and female ; but the essential difference is in the relation of the two sexes to the production of the genoblasts; the male produces the spermatozoa, the female the ova, and in this lies the whole essence of the sexual differentiation ; all other distinctive morphological and physiological traits of men and women are secondary. Thus the structure and functions of the genital ducts, of the uterus, mammary glands, etc., though eminently characteristic of the sexes, in man are not from a biological point of view fundamental.

As we ascend the animal scale there is an increasing divergen( e between the sexes, owing to the increasing adaptation to the reproductive functions. It is generally l)elieved that the primitive condition is hermaphroditic, and that the female is an individual in which the power of producing male elements is lost, and a male an individual in which the power of producing female elements is lost. In a certain sense this conception appears true, for in the embryo there is an indifferent stiige in which the sexual glands are already differeiitiateil, but in which the future sex is uurecognizable ; subseciuently by unknown factors the sexual gland is converted into an ovary or a testis. In some cases, as in certain teleosts and in the snails, the sexual glands develop lK>th ova and si)ermatozoa. These facts suggest that the primitive sexual gland is potentially hermaphroditic. It is to be remembered, however, that if hermaphroditism were the primitive form we should expect to find the lowest metazoa hermaphroditic; but this is no* the case either with all Coelenterata or all si)onges, although it is the case in some higher classes of the animal kingdom — as, for instance, the trematode worms and pulmonate gasteropods. These and other considerations have led me to the h3'pothe8i8 that primitively each individual animal is sexually indifferent when young, and becomes either male or female when adult; by a secondarv modification in certain forms the individual becomes both male and female. This is contrary to the prevalent opinion that the hermaphroditic condition is the primitive one.

VI. Heredity

In regard to the process of hereditary transmission there are two theories, each of which appears in several modifications- Ist, the theory of pangenesis; 2d, the theory of germinal continuity, The latter does, the former does net, appear to me to conform to our present knowledge.

Pangenesis. — The theory of pangenesis was first formulated by Darwin, thoup;h it had been crudely foreshadowed by Buffon, Bonnet, and Herbert Spencer. The following quotation from Darwin's "Animals and Plants under Domestication" (Amer. edit., 1868, II., 448, 449) gives his statement of his theory: "I have now enumerated the chief facts which every one would desire to connect by some intelligible bond. This can be done, as it seems to me, if we make the following assumptions ; if the first and chief one be not rejected, the others, from being supported by various physiological considerations, will not appear very improbable. It is almost imiversally admitted that cells or the units of the body propagate themselves by self-division or proliferation, retaining the same nature and ultimately liecoming converted into the various tissues and substances of the body. But besides this means of increase I assume that cells, before their conversion into completely passive or "formmaterial," throw off minute granules or atoms, which circulate freely throughout the system, and when supplied with proper nutriment multiply by self -division, subsequently Incoming developed into cells, like those from which they were derived These granules, for the sake of distinctness, may be calleil cell-gemmules, or, as the cellular theorj" is not fully established, simply gemmules. They are supposed to be transmitted from the parents to the offspring, and are generally developed in the generation which immediately succeeds, but are often transmitted in a dormant state during many generations and are then developed. Their development is supposed to depend on their union with other partially developed cells or gemmules, which precede them in the regular course of growth. Why I use the term union will be seen when we discuss the direct action of pollen on the tissues of the Inother-plant. Gemmiiles are supposed to be thrown off by every cell or unit, not only during the adult state, but during all stages of development. Lastl}', I assume that the gemmules in their . dormant state have a mutual affinity for each other, leading to their aggregation either into buds or into the sexual elements. Hence, speaking strictly, it is not the reproductive elements nor the buds which generate new organisms, but the cells themselves throughout the body. These assumptions constitute the provisional hypothesis which I have called Pangenesis."

This hypothesis is the suggestion of a masterly mind, and, as a succinct and comprehensive expression of the facts of heredity, must always commancl admiration. But the real worth and real significance of the hypothesis have not been grasped by those who have tried to better it; its value is not in explaining, for it does not do that, but in expressing heredity in hyi)othetical terms, which are at once suggestive and comprehensible. Haeckel, in an amusing pamphlet,* which no competent critic can assign the slightest value to, asserted that the gemmules are rhythmical vibrations, but he gives no reasons to justify his opinion. Elsberg has also written on the subject in the Proc. Amer. Assoc. Adv. Sci., XXV , 178, and cites there earlier writings of his own.f

Brooks' modification, 76 . 1 ,of the theory of pangenesis well deserves consideration, although the subsequent progress of biology does not lead me to think it felicitous; but we can now recognize it as a stop toward Nussbaum's valuable theory of germinal continuity, and also toward Weismann's conception that sexual reproduction has for its object the maintenance of variability Brooks' theory is advocated in his book on " Heredity" (Baltimore, 1871)) ; he states it succinctly as foUow^s • I " This paper proposes a modification of Darwin's hypothesis of the same name (pangenesis), removing most of its difficulties, but retaining all that is valuable. According to the hypothesis in its modified form, characteristics which are constitutional and already hereditary are transmitted by the female organism by means of the ovum ; while new variations are transmitted by gemmules, which are thnnvn off by the varying phsyiological units of the body, gathered up by the testicle and transmitted to the next generation by impregnation." If this theory was tenable, there should be — to mention a single objection — little variation in individuals produced by parthenogenesis , and they ought always to be females, whereas they are sometimes males. There remains not a new theor}' of pangenesis, but the valuable suggestion that the maternal influence causes less variability than the paternal. I am, however, strongly disinclined to anticipate the confirmation of this suggestion, especially because the males are not more variable than the females, as we should expect. I have some extensive statistics which show

  • E. ITaeckel "PoretreneKi** der Plastidule." B»»rlln 1^76 For some criticisms which, considering the chamcter of this pamphlet, are very gentle see Ray LAnkester in Nature July 13th. 1876, xlv 235-338

t The penisal of Elsherg's article has not enabled me to recojmiie anything novel except the subKtitution of the term plastidule for gemmule. used Uy Darwin and sjiecrulations as to the composition of plastidules as if he were groping after the conception of the unicella of Nftgeli, with which he was apparently unacquainted

t Proc Amer Assoc Sc. Buffalo IWti p 177 abstract of a paper read before the section of natural history that in mammals, at least, there are no essential differences between the sexes in variability. Even if Bnx)ks' thesis should be established it would prove only that the inheritanee from the mother is stronger than from the father, and there would lack reasons for his abstruse hypothesis.

The theory of pangenesis is to be resigncnl, not so much on account of the direct arguments against it, as on account of the accumulation of evidence in favor of the theory of genninal continuity.

Q^rminal Continuity. — There are various theories to be considere<l imder this head ; but they all have in common the conception that there is a formative force in organisms — that the force depends ujxm a special material substratum, and that some of the supply of that substratum is given by the parent to the sexual elements it pr<xhices.

The first important step toward the substitution of a new theory rice piingenesis was taken, so far as I am aware, by Moritz Nussb^uim, whose memoirs, 80.1, 84.2, on the differentiation of sex deserve great attention. August Weismann* has adopted Nussbaum\s conception and defended it with insistent energy, adding also several modifications. Xussbaum pointed out that there is noteworthy evidence in the development of various animals tending to show that the germinal cells, from which the sexual products arise, are separateil off verv' early from the other cells of the embryo and undergo very little alteration. Hence he concluded that some of the germ substance is directly abstracted from the developing ovum and preserved without essential alteration to l)ecome, by giving rise to the sexual elements, the germ substiuice of another generation. Weismann insists uyion the corollary that the whole nature of the animal or plant depends upon its germinal substance {Keimplasma) ^ and that the reason why the offspring is like the parent is that in every genoblast some of the germinal matter is preserved imchanged. He calls this view the theory of the continuity of the germ-plasma. He follows Xussbaum also in emphasizing the fact that this theory is inconsistent with the theory of pangenesis and with the theory that par€»ntal characteristics acquired through the influence of external causes are transmissible to the offspring. On these two jx>ints Weismann's second and third papers [ire especially important. Xussbaum and Weismann lay great stress \\\K)n the separation of the cells of the embryo into two kinds: 1, the germ-cells, which are converted into the sexual elements; 2, the somatic- cells, which constitute the body of the organism. The germ-cells descend directly from the ovum, ac<X)rding to Weismann, who has carried his speculations to a great extreme, and undergo little alteration, so that they have (in suspension) the |X)wer to produce a whole organism, which the somatic-cells do not have. It is im|x)ssible to agree to this extraordinary view.

• \\>i8mann*8 first i>ap<M- was rend lK»forp the Unlversitv of Freihurp as a ProreotoratJjrede. and was nuhlisliHl in iMiiiiphlet f«»rm at Jena in 1HK3. 83.1. A second )>ai)er was rearl before the (tennan NaturforHc'her\er»iamMjhjn»f in 1HH5. ami anpeare<l in the Taceblutt of that Assivlation. it was subsequently amplified and i-epubli8he<l, As.l. A thinl paper. 86.3, was likewise addressetl to the Naturfors<'herversaniinIunjf in 1886, and piiblisherl at Jena the same year A notice of this last is piven by Kollmann. Biol. Cbl.. v.. 673 and TO 5. At thesame meetinir of the Xaturforacher. R. Virchow also delivtred an address rsee Virchow's Arch., ciii 1 305 413 and alMtract in Biol. Cbl., vi. v/T. rj9, 161.) in which he attacked Weismann. To kollmann and Virchow Weismann has rei>Iied in Biolo^. Centralbl., vi. 38.

Minot, 70, has expressly emphasized the fact that the formative force is certainly a diffused one, as is amply proven by the processes of regeneration, by the phenomenon of duplication of parts, and by asexual reproduction, since in all ttiese cases the formation of a part or the whole of the organism proceeds without the participation of the sexual elements. KoUiker, also, 85. 1, 44-40, clearly demonstrates that a sharp division between germ-cells and somatic-cells cannot be maintained. The same position has been adopted by Whitman, 87.3, and, of course, by many others. It is to he further remembered that the cells for the different organs of the body are all set apart very early indeed, and in the case of vertebrates the germcells are among the very latest to become distinguishable; thus the nerve-cells, muscle-cells, notochord-cells, etc., etc., all can be seen to precede the germ-cells in their differentiation. Weismann's assumption that the germ-cells are set apart specially early is simply false; all the organs have their cells set apart early, and that too while they are in the embryonic condition; and it is not true that the germ-cells differ essentially as to their mode of origin or differentiation from the so-called somatic-cells. The early divergence of the cells according to the organs or parts they are destined for was pointed out explicitly Ijy W. His many years ago, 74.1, 18. 19. Weismann's error consists in attributing a peculiar significance to a fact by connecting it only with the development of the sexual elements, whereas it is a fact common to all parts of the body. All, therefore, of Weismann's further speculations as to the difference between germ plasma and " histogenes plasma" are without f oimdation.

Nageli was probably the first to reach the definite conception of a material basis of heredity, to which basis he gave the name of idioplasma. This idioplasma is essentially identical, it seems to me, with Weismann's Keimplasma. Nageli's views are presented very fully in a large, abstruse, and little-studied volume, of which a useful abstract has been given by KoUmann {Biol. Cbl., IV., 488, 517). Nageli is led to the theory that there are in every living cell two substances, one of which, the idioplasma, alone carries on the function of hereditary transmission, while the other, the nutritive plasma (Ndhrplasma) is the seat of the remaining functions. In other words, Nageli put forward in a definite form the theory of germinal continuity, for he assumes the formative force to reside in a specific material substrattim, which reproduces and perpetuates itself, occurs throughout the organism, and, therefore, in the genital products also. The argument in support of this theory is very able, and well deserves the cordial praise which KoUiker and others have bestowed upon it.

Nageli did not specify what constituent of the cell corresponds to his idioplasma. O. Hertwig, 85.1, was the first to indicate the nucleus as the organ of heredity, and this view has been ably defended by KoUiker, 85.1, Strassburger, and others. This notion rests upon the consideration of — 1st, various facts which suggest that the nucleus has special control over the organization of the cell ; 2d, the probability that impregnation consists essentially in the fusion of the pronuclei; 3d, the development of the spermatozoon from the nucleus. That the nucleus presides over the cells is naturally suggested by the phenomena of cell-division, especially indirect division (karyokinesis, mitosis) , for during the process the nucleus leads the way, and its visible alteration preced€*s that of the protoplasm ; the astral rays both during karyokinesis and those around the pronuclei during impregnation may be interpreteil as results of nuclear control. The opposite conception that the protoplasm leads has not lacked defenders (see Auerbach, Biitschli, 76.1, Nussbaum, 86.1, 504, and Whitman, 88.1). I may point out that in interpreting the observations bearing upon this discussion, we must not forget that the nucleus and protoplasm are interdependent, neither being able to maintain its existence ])ermanently without the other. The fact/' says Minot, 85, 125, "that the visible alteration of the protoplasm in a certain rare case comes before that of the nucleus shows that the protoplasm probabl}' has an active r<3le in cell-division ; but since even then its arrangement depends on the j>osition of the nucleus, the evidence of the superiority of nuclear control is, I think, not affected." On the other hand, there are many observations, which may be interpreteil as i)ro()fs, that the nuclei have a regulating power over the cells, especially as regards their division and organization. A few of these may be instanced: 1st. After a cell is formed, its nucleus enlarges first, and the cell-boily follows it in growth. 2d. KoUiker, in his paper, 85.1, on heredity (p. 21» ff.), discusses the relation of nuclei to growth very fully and ably. The great extent of his learning has enabled him to present the manifold aspects of the question more thoroughly than any other writer. His argumentation seems to me so satisfactory that it does not re<iuire the weight of his ^reat authority to establish the conclusion that without nuclei there 18 no growth. Of this the most faith-comi)elling evidence is offered by the important experiments jf Nussbaum and Gruber,* who found that when unicellular animals are artificially divided, the fragments containing nuclei continue to grow, while pieces without nuclei die off. 3d. The large unicellular Thallophytes, such as Caulerpa and Codium, become multinuclear l)efore they attain their tidult size. Further illustrations are given by Kolliker (/. c*., pp. TJ, 20). 4th. Perhaps the most striking demonstration of the importance of the nucleus is afforded by the ex j:)eri mental alteration of the plane of division of the ovum. Pfiuger, 83.6, showed that the plane of the first division of the ovum is altered by tilting the o\aun before the division begins, and keeping it in the same ix)sition during division; normally the plane ]>asses through the white ix)le, but when the ovum is fastened in an o])lique position, the ])lano is not in the axis of the ovum, but in the line of gravity. Boni,t 84.3, has continued these remarkable ex]>eriments, and discovered that the nucleus changes its position when the ovum is kept tilted, and that the site of the nucleus determines the plane of division of the ovum. The second and third points (the im|)ortance of the pronuclei and tho nuclear origin of the spermatozoon) have been sufficiently elucidated in previous divisions of this chapter. Now, it is obvious, since qualities may be inherited from the rather, that the nucleus alone can furnish the means of transmission from parent to offspring ; and, since it can accomplish this on the paternal side, it is probable that it can do as much on the maternal side — an assumption against which no evidence has been brought forward ; hence the hypothesis that the nucleus is the organ of hereditary transmission. For criticism of this view see J. Frenzel, 86.5, p. 89, whose arguments have been controverted by Minot, 85, 127.

  • Science, vol. vi . p. 4. S*»«» al*4<) NiiKsbaum's lat^r ivuii^rT in thoArchiv fiir inikntskop. Anat., xxvi.p 4H5. NiisHl)aiiiii also citt's Fr. Schmitz's pxiMTiriu'nts on the artificial division of plants S<!hmitz's nai)er I have not seen: it was published in \^\K in the Ftvtsohrift lier natiirxorschenden f?e»ellsc*haft zu HaJle.

t I have not seen ihe original There is an al»stra<*t in Ilofnianu un«l SchwaU»e's Jahresbericht for 1W4 p 444

We may go one step farther : Since the chromatin is the cliaracteristic of the nucleus, and since spermatozoa in some cases consist almost exclusively of chromatin, it is probable, as maintained by Minot, 85, 127, that chromatin is the essential factor in the function of heredity. It is my conviction that the hypothesis of pangenesis, both in its original form and in all its subsequent modifications, has been definitely set aside. In its place we have the theory that the nature of germ, i. e., of the impregnated ovum, is the same over and over again, not because there is in each case a similar collocation of gemmules or plastidules, but because the chromatin perpetuates itself so that the same kind of chromatin is found in the one generation as in the generations preceding it and following it. The child is like the parents because its organization is regulated by not merely sinu'ku\ but by some of the sanie^ chromafin as that of the parents. Perhaps instead of chromatin" we ought to say, in order to avoid an unjustifiable explicitness, "nuclear substance."

The validity of this hypothesis remains for the future to decide. There is one general objection to it — that of connecting a special function with a special substance, which is against the general conception of vital functions as the resultants of interlocking activities extending throughout each celL Compare the remarks a propos of the theory of sex, ante^ p. 70. The objection is, to my mind, a real and very serious one.

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