Book - Comparative Embryology of the Vertebrates 2-4

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Nelsen OE. Comparative embryology of the vertebrates (1953) Mcgraw-Hill Book Company, New York.

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Part II - The Period of Fertilization

Part II - The Period of Fertilization: 4. Transportation of the Gametes (Sperm and Egg) from the Germ Glands to the Site where Fertilization Normally Occurs | 5. Fertilization

Transportation of the Gametes

(Sperm and Egg) from the Germ Glands to the Site where Fertilization Normally Occurs

A. Introduction

1. Activities of the Male and Female Gametes in Their Migration to the Site of Fertilization

The first step in the actual process of fertilization and the reproduction of a new individual is the transportation of the mature gametes from the place of their development in the reproductive structures to the area or site where conditions are optimum for their union (fig. 98). This transport is dependent upon the development of the proper reproductive conditions in the male and the female parent — a state governed by sex hormones. That is to say, the sex hormones regulate the behavior of the parents and the reproductive ducts in such a way that the reproductive act is possible.

The transport of the female gamete to the site of fertilization is a passive one, effected by the behavior of the reproductive structures. Also, the transportation of the sperm within the confines of the male tract largely is a passive affair. However, outside of the male reproductive tract, sperm motility is a factor in effecting the contact of the sperm with the egg. Not only is sperm motility a factor in the external watery medium of those species accustomed to external fertilization, but also to some degree within the female genital tract in those species utilizing internal fertilization. However, in the latter case, sperm transport is aided greatly by the activities of the female genital tract.

B. Transportation of the Sperm Within the Male Accessory Reproductive Structures

1. Transportation of Sperm from the Testis to the External Orifice of the Genital Duct in the Mammal

Sperm transport within the male genital tract of the mammal is a slow process. It might be defined better by saying that it is efficiently slow, for the ripening process of the sperm described in the previous chapter is dependent upon a lingering passage of the sperm through the epididymal portion of the male genital tract.



Fig. 98. Sites of normal fertilization (x) in the vertebrate group. (A, C) Vertebrates below mammals. (B) Mammalia.


a. Possible Factors Involved in the Passage of the Seminal Fluid from the Testis to the Main Reproductive Duct

1) Accumulated Pressure Within the Seminiferous Tubules. The oozing of sperm and seminal fluid from the seminiferous tubules through the reU tubules into the efferent ductules of the epididymis possibly may be the resul of accumulated pressure within the seminiferous tubules themselves. Thi: pressure may arise from secretions of the Sertoli cells, the infiltration of fluid! from the interstitial areas between the seminiferous tubules, and by the addition of sperm to the contents of the tubules. As the seminiferous tubule is blind at its distal end, increased pressure of this kind would serve efficiently to push the contained substance forward toward the efferent ductules connecting the testis with the reproductive duct.


2) Activities Within the Efferent Ductules of the Testis. The time required for sperm to traverse the epididymal duct in the guinea pig is about 14 to 16 days. However, when the efferent ductules between the testis and the epididymal duct are ligated, the passage time is increased to 25 to 28 days (Toothill and Young, ’31). The results produced by ligation of the ductuli effe rentes in this experiment suggest: (a) That the force produced by the accumulation of secretion within the seminiferous tubules and adjacent ducts tends to push the sperm solution out of the seminiferous tubules into the ductuli efferentes and thence along the epididymal duct, and/or (b) at least a part of the propulsive force which moves the contents of the seminiferous tubules through the rete tubules and efferent ductules and along the epididymal duct arises from beating of cilia within the lumen of the efferent ducts. The tall cells lining the latter ducts possess cilia which beat toward the epididymal duct. As the sperm and surrounding fluid reach the efferent ductules, the beating of these cilia would propel the seminal substances toward the epididymal duct.

b. Movement of the Semen Along the Epididymal Duct

1) Probable Immotility of the Sperm. The journey through the epididymal duct as previously indicated is tedious, and secretion from the epididymal cells is added to the seminal contents (fig. 99). Sperm motility evidently is not a major factor in sperm passage along the epididymal portion of the reproductive duct, as conditions within the duct appear to suppress this motility. It has been shown, for example (Hartman, ’39, p. 681), that sperm motility increases for trout sperm at a pH of 7.0 to 8.0, in the mammals a pH of a little over 7.0 seems optimum for motility for most species, while in the rooster a pH of 7.6 to 8.0 stimulates sperm movements. On the other hand, an increase of the CO 2 concentration of the medium raises the hydrogen ion concentration of the suspension. The latter condition suppresses sperm motility and increases the life of sea-urchin sperm (Cohn, ’17, ’18). These facts relative to the influence of pH on the motility of sperm suggest that motility during the slow and relatively long epididymal journey — a journey which may take weeks — apparently is inhibited by the production of carbon dioxide by the large aggregate of sperm within the lumen of the epididymal duct, a condition which serves to keep the spermatic fluid on the acid side. This suppressed activity of the sperm in turn increases their longevity. The matter of sperm motility within the epididymal duct, however, needs more study before definite conclusions can be reached relative to the actual presence or absence of motility.


Fig. 99. Human epididymal cells. (Slightly modified from Maximow and Bloom: A Textbook of Histology, Philadelphia, W. B. Saunders Co.) These cells discharge secretion into the lumen of the epididymal duct. Observe large, non-motile stereocilia at distal end of the cells.


2) Importance of Muscle Contraction, Particularly of the Vas Deferens.

If sperm are relatively immobilized during their passage through the epididymal duct by the accumulation of carbon dioxide, we must assume that their transport through this area is due mainly to the activities of the accessory structures together with some pressure from testicular secretion and efferentductule activity as mentioned above. Aside from the forward propulsion resulting from the accumulation of glandular secretion within the epididymal duct, muscle contraction appears to be the main factor involved in effecting this transport. The epididymal musculature is not well developed, and muscle contraction in this area may be effective but not pronounced. However, added to the contracture of the epididymal musculature is the contraction of the well-developed musculature of the vas deferens (fig. 100). During sexual stimulation this organ contracts vigorously, producing strong peristaltic waves which move caudally along the duct. The activity of the vas deferens may be regarded as a kind of “pump action” which produces suction sufficient to move the seminal fluid from the caudal portions of the epididymis, i.e., from the cauda epididymidis into the vas deferens where it is propelled toward the external orifice. Furthermore, the removal of materials from the cauda epididymidis would tend to aid the movement of the entire contents of the epididymal duct forward toward the cauda epididymidis. From this point of view, the vas deferens is an efficient organ for sperm transport, while the epididymal duct functions as a nursery and a “storage organ” for the sperm (see Chap. 1). Some sperm also are stored in the ampullary portion of the vas deferens (fig. 101 ), but this storage is of secondary importance inasmuch as sperm do not retain their viability in this area over extended periods of time.


(4) the possibility of a weak sperm motility aiding the advance of the sperm through the body of the epididymis must not be denied;

(5) the vigorous pumping action of the vas deferens, especially during the stimulation attending ejaculation, serves to transport the sperm from the “epididymal well” (the cauda epididymidis) through the vas deferens to the external areas.

2. Transportation of Sperm in Other Vertebrates with a Convoluted Reproductive Duct

The transportation of sperm in other vertebrates which possess an extended and complicated reproductive duct similar to that of the mammal presumably involves the same general principles observed above (fig. 105 A, B). However, certain variations of sperm passage exist which are correlated with structural modifications of the accessory reproductive organs. For example, the reproductive duct may be somewhat more tortuous and complicated in some instances, such as in the pigeon, turkey, and domestic cock (figs. 102, 105B). That is, the entire deferent duct extending from the epididymis caudally to the cloaca may be regarded as a sperm-storage organ, as sperm may be collected in large numbers all along the reproductive duct. As the cock is capable of effecting repeated ejaculations over an extended period of time.


Fig. 101. Portion of a cross section of the ampullary region of the ductus deferens in man. Observe gland-like outpouchings of the main lumen and character of mucosal folds. Surrounding the lumen may be seen the highly muscularized walls of the ampullary area.


Fig. 102. Reproductive and urinary structures of the adult Leghorn cock. Observe that the vas deferens is a much convoluted structure. (After Domm: In Sex and Internal Secretions, by Allen, et al., Baltimore, Williams & Wilkins, 1939.)

each contraction of the caudal portion of the deferential duct during sperm discharge serves to move the general mass of seminal fluid posteriad in a gradual manner. The reproductive conditions present in the cock fulfill the requirements of a continuous breeder capable of serving many individual females. It is to be observed in this connection that Mann (’49) gives the amount of ejaculate in the cock as 0.8 cc., highly concentrated with sperm.


Fig. 104. Modifications of the fins of male fishes with the resulting elaboration of an intromittent organ. (A) Catnhusia affittix. (B) Ventral view of pelvic fins of Squalus acanthias. (C) Dorsal view of left fin to show genital groove in intromittent structure.


Another variation found in certain birds is the presence of a seminal vesicle located at the caudal end of the reproductive duct. This outgrowth is a spermstorage organ and is not comparable to the secretory seminal vesicle found in mammals. Such seminal vesicles are found in the robin, ovenbird, wood thrush, catbird, towhee, etc. These structures enlarge enormously during the breeding season, but in the fall and winter months they shrink into insignificant organs (Riddle, ’27). It is apparent that the seminal fluid is moved along and stored at the distal (posterior) end of the reproductive duct in these species. Other birds, such as the pigeon and mourning dove, lack extensively developed seminal vesicles, but possess instead pouch-like enlargements of the caudal end of the reproductive duct when the breeding season is at its maximum.

In many lower vertebrates which practice internal fertilization, large seminal vesicles or enlargements of the caudal end of the reproductive duct are present. Such conditions are found in the elasmobranch fishes. These structures act as sperm-storage organs during the breeding season.

3. Transportation of Sperm from the Testis in Vertebrates Possessing a Relatively Simple Reproductive Duct

In forms such as the frog, toad, and hellbender (figs. 9, 105C), the pressure within the seminiferous tubules of the testis associated with contractions of the reproductive duct serve to move the sperm along the reproductive duct. At the time of spawning, a copious discharge of sperm is effected. In teleost fishes, a general contraction of the testicular tissue and the muscles of the abbreviated sperm duct propel the sperm outward during the spawning act (fig. 105D). In teleosts, sperm are stored in the testis, or as in the perch, large numbers may be accommodated within the reproductive duct (fig. 105D) . Slight motility also may be a factor in effecting sperm transport down the reproductive duct in the lower vertebrates.

C. Transportation of Sperm Outside of the Genital Tract of the Male

1. Transportation of Sperm in the External Watery Medium

In most teleost fishes and in amphibia, such as the frogs and toads, and the urodeles of the families Hynobiidae and Cryptobranchidae (possibly also the Sirenidae), fertilization is external and sperm are discharged in close proximity to the eggs as they are spawned. Many are the ways by which this relationship is established, some of which are most ingenious (fig. 103). Sperm motility, once the watery medium near the egg is reached, brings the sperm into contact with the egg in most instances. However, exceptional cases are present where the sperm are “almost completely immobile,” such as in the primitive frog, Discoglossus (see Hibbard, ’ 28 ). Here the sperm must be deposited in close contact with the egg at the time of spawning. In fishes which lay pelagic eggs (i.e., eggs that float in the water and do not sink to the bottom), the male may swim about the female in an agitated manner during the spawning act. This behavior serves to broadcast the sperm in relation to the eggs.


Fig. 105. Various types of reproductive ducts in male vertebrates. The possible activities which transport the sperm along the ducts are indicated. (A) Mammalian type (B) Bird, urodele, elasmobranch fish type. (C) Frog type. (D) Teleost fish type.



Fig. 106. Brood pouch in the male pipefish. (A) Longitudinal view with left flap pulled aside to show the developing eggs within the pouch. (B) Transverse section to show relation of eggs to the pouch and dorsal region of the tail.


2. Transportation of Sperm in Forms where Fertilization of the Egg is Internal

a. General Features Relative to Internal Fertilization

1) Comparative Numbers of Vertebrates Practicing Internal Fertilization.

Of the 60,000 or more species of vertebrates which have been described, a majority practice some form of internal fertilization of the egg. Internal fertilization, therefore, is a conspicuous characteristic of the reproductive phenomena of the vertebrate animal group.

2) Sites or Areas where Fertilization is Effected. The fertilization areas (fig. 98) for those vertebrates which utilize internal fertilization are:

( 1 ) the lower portions of the oviduct near or at the external orifice,

(2) the oviduct, especially its upper extremity,

(3) possibly the peritoneal cavity,

(4) the follicles of the ovary, and

(5) the brood pouch of the male (figs. 98, 106).

Though the exact place where internal fertilization occurs may vary considerably throughout the vertebrate group as a whole, the specific site for each species is fairly constant.

3) Means of Sperm Transfer from the Male Genital Tract to That of the Female. In those fishes adapted to internal fertilization, sperm transport from the male to the female is brought about by the use of the anal or pelvic fins which are modified into intromittent organs (fig. 104). In the amphibia two genera of Anura are known to impregnate the eggs within the oviduct of the female. In the primitive frog, Ascaphus truei, the male possesses a cloacal appendage or “tail,” used to transport the sperm from the male to the female, and the oviducts become supplied with sperm which come to lie between the mucous folds (Noble, ’31). (See fig. 107.) In East Africa, in the viviparous toad, Nectophrynoides vivipara, fertilization is internal, and the young, a hundred or more, develop in each uterus. (See Noble, ’31, p. 74.) Just how the sperm are transmitted to the oviduct and whether fertilization is in the lower or upper parts of the oviduct in this species is not known.

In contrast to the conditions found in most Anura, the majority of urodele amphibia employ internal fertilization. In many species the male deposits a spermatophore or sperm mass (fig. 10). The jelly-like substance of the spermatophore of the salamanders is produced by certain cloacal or auxiliary reproductive glands. The spermatophore may in some species be picked up by the cloaca of the female or in other species it appears to be transmitted directly to the cloaca of the female from the cloaca of the male. As the spermatophore is held between the lips of the cloaca of the female, it disintegrates and the sperm migrate to and are retained within special dorsal diverticula of the cloacal wall known as the spermatheca (Noble and Weber, ’29) (fig. 108).


Fig. 107. Intromittent organ of the tailed frog of America, Ascaphus iruei. (After Noble, ’31.) (A) Cloacal appendage. (B) Ventral view of same. (C) Fully distended appendage, showing spines on distal end. Opening of cloaca shown in the center.



Fig. 108. Diagrammatic sagittal sections of the cloacas of three salamanders, showing types of spermatheca. (A) Necturus. (B) Amhystoma. (C) Desmognathus. (Redrawn from Noble, ’31.)


In the male of the gymnophionan amphibia, a definite protrusiblc copulatory organ is present as a cloacal modification, and fertilization occurs within the oviducts (fig. 109). Extensible copulatory organs are found generally in reptiles and mammals, and are present also in some birds, such as the duck, ostrich, cassowary, emu, etc. In most birds the eversion of the cloaca with a slight protrusion of the dorsal cloacal wall functions very effectively as a copulatory organ.

b. Methods of Sperm Transport Within the Female Reproductive Tract

1) When Fertilization Is in the Lower or Posterior Portion of the Genital Tract. In many of the urodele amphibia, fertilization is effected apparently in the caudal areas of the female genital tract or as the egg passes through the cloacal region. It is probable in these cases that sperm motility is the means of transporting the sperm to the egg from the ducts of the spermatheca or from the recesses of the folds of the oviduct.

2) When Fertilization Occurs in the Upper Extremity of the Oviduct. In several species of salamanders, fertilization of the egg and development of the embryo occur within the oviduct. Examples are: Salamandra salamandra, S. atra, Hydromantes genei and H. italicus, all in Europe, and the widely spread neotropical urodele, Oedipus. The latter contains many species. The exact region of the oviduct where fertilization occurs is not known, but presumably, in some cases, it is near the anterior end. Weber (’22) suggests that fertilization may occur normally in the peritoneal cavity of Salamandra atra. In these instances, the method by which the sperm reach the fertilization area is not clear. It is probable that motility of the sperm themselves has much to do with their transport, although muscular contraction and ciliary action may contribute some aid.

On the other hand, studies of sperm transport in the female genital tract in higher vertebrates have supplied some interesting data relative to the methods and rate of transport. In the painted turtle, Chrysemys picta, sperm are deposited within the cloacal area of the female during copulation; from the cloaca they pass into the vaginal portion of the oviduct and thence into the uterus. It is possible that muscular contractions, antiperistaltic in nature, propel the sperm from the cloaca through the vagina and into the uterus. It may be that similar muscle contractions propel them through the uterus up into the albumen-secreting portions of the oviduct, or it is possible that sperm motility is the method of transport through these areas. However, once within the albumen-secreting section of the oviduct, a band of pro-ovarian cilia (i.e., cilia which beat toward the ovary) (fig. IlOA, B) appears to transport the sperm upward to the infundibulum of the oviduct (Parker, *31). Somewhat similar mechanisms of muscular contraction, antiperistaltic in nature, and beating of pro-ovarian cilia are probably the means of sperm transport in the pigeon and hen (Parker, ’31). Antiperistaltic muscular contractions are known to be possible in the hen (Payne, ’14). Active muscular contractions are suggested, as sperm travel upward to the infundibulum of the oviduct in about one and one-half hours in the hen.


In the rabbit, antiperistaltic contractions of the cervix and body of the uterus at the time of copulation pump or suck the sperm through the os uteri from the vagina and transport them into the uterus at its cervical end (Parker, ’3 1 ) . This transportation occupies about one to three minutes. Passage through the body of the uterus to the Fallopian tube occurs in one and one-half to two hours after copulation. It is not clear whether sperm motility alone or sperm motility plus uterine antiperistalsis effects this transportation. The transport of the sperm upward through the Fallopian tube to the infundibular region takes about two hours more. The behavior of the uterine (Fallopian) tube is somewhat peculiar at this time. Churning movements similar to that of the normal activity of the intestine are produced. Also, temporary longitudinal constrictions of the wall of the tube produce longitudinal compartments along the length of the tube. Within these compartments cilia beat vigorously in an abovarian direction (i.e., away from the ovary). The general result of these activities is a thorough mixing and churning of the contents of the tubes. At the same time these movements succeed in transporting the sperm up the tube to the infundibular area. The entire journey through the uterus and Fallopian tube consumes about four hours (Hartman, ’39, pp. 698-702; Parker, ’31).

Sperm transport through the female genital tract in the rabbit occupies a relatively long period of time compared to that which obtains in certain other mammalian species. The journey to the infundibular area of the Fallopian tube takes only 20 minutes in the majority of cases in the ewe, following normal service by the ram. The rate of sperm travel toward the ovaries is approximately four cm. per minute (Schott and Phillips, ’41). The passage time through the entire female duct may be considerably less than this in the guinea pig, dog, mouse, etc. (Hartman, ’39, p. 698). It is probable that the latter forms experience antiperistaltic muscular contractions of the uterine cervix, uteri, and Fallopian tubes, which propel the sperm upward to the infundibular region, the normal site of fertilization.

In the marsupial group the lateral vaginal canals complicate the sperm transport problem. In the opossum, the bifid terminal portion of the penial organ (fig. 1 14A) probably transmits the sperm to both lateral vaginal canals simultaneously, where they are churned and mixed with the taginal contents. From the lateral vaginal canals the sperm are passed on to the median vaginal cul-de-sac. From this compartment they travel by their own motive power or are propelled upward through the uterus and Fallopian tubes to the infundibular area of the latter (figs. 34, 35, 114).

The foregoing instances regarding sperm transport in the female mammal involve active muscle contractions presumably mediated through nerve im


Fig. 111. Dorsal view of anterior end of uterine horn of the common opossum, Didelphys virginiana, showing relation of ovary to infundibulum.



Fig. 113. Open body cavity of adult female of Rana pipicns, showing distribution of cilia and ostium of oviduct. (Slightly modified from Rugh, ’35.)

5ulses aroused during the reproductive act or orgasm together with the actual Dresence within the reproductive tract of seminal fluid. However, this nervenuscular activity is assuredly not the only means of sperm transport although t may be the more normal and common method. A slower means of trans)ort, that of sperm motility, plays an important role in many instances. This s suggested by such facts as fertility being equal in women who experience 10 orgasm during coitus compared to those who do; proven fertility in rabbits ind dogs whose genital tracts are completely de-afferented by spinal section; ind conception by females artificially inseminated intra vaginum. (See Hartnan, ’39, p. 699.) Moreover, Phillips and Andrews (’37) have shown that

at sperm injected into the vagina of the ewe along with ram sperm lag behind he ram sperm in their migration upward in the genital tract. That is, the ibnormal environment of the genital tract of the ewe in which the rat sperm were placed may have affected their motility, as well as their ability to survive. (See Yochem, ’29.)

The above data suggest relationships in many of the vertebrates which doubly assure that sperm will reach the proper site for fertilization in the oviduct. One aspect of this assurance is the physiological behavior of the anatomical structures of the oviduct, which may express itself by ciliary beating in some instances or, in other cases, by muscle contraction. On the other hand.


Fig. 114. Bifid penis of the male opossum; diagram of female reproductive tract. (A) Extended penis. (After McCrady, Am. Anat. Memoirs, 16, The Wistar Institute of Anatomy and Biology, Philadelphia.) (B) Female reproductive tract.


if this method fails or is weakened, sperm motility itself comes to the rescue, and sperm are transported under their own power.

In view of the above-mentioned behavior of the oviduct in transporting sperm, it is important to observe that the estrogenic hormone is in a large way responsible for the activities of the oviduct during the early phases of the reproductive period and, consequently, influences the conditions necessary for sperm transport. It enhances this process by arousing a state of irritability and reactivity within the musculature of the uterus and the Fallopian tubes. It also induces environmental conditions which are favorable for sperm survival within the female genital tract.

3) When Fertilization Occurs in the Ovary. In certain viviparous fishes the egg is fertilized in the ovary (e.g., Gambusia affinis; Heterandria jormosa). (See Turner, ’37, ’40; Scrimshaw, ’44.) As the sperm survive for months in the female tract, sperm transport is due probably to the movements of the sperm themselves. Motility evidently is a factor in the case of the eutherian mammal, Ericulus, where ovarian fertilization presumably occurs according to Strauss, ’39.


D. Sperm Survival in the Female Genital Tract

The length of life of sperm in the female genital tract varies considerably in different vertebrates. In the common dogfish, Squalus acanthias, and also in other elasmobranch fishes, sperm evidently live within the female genital tract for several months, and retain, meanwhile, their ability to fertilize. In the ordinary aquarium fish, the guppy (Lebistes), sperm may live for about one year in the female tract (Purser, ’37). A long sperm survival is true also of the “mosquito fish,” Gambusia. Within the cloacal spermatheca of certain urodele amphibia, sperm survive for several months. Within the uterus of the garter snake they may live for three or more months (Rahn, ’40), while in the turtle, Malaclemys centrata, a small percentage of fertile eggs (3.7 per cent) were obtained from females after four years of isolation from the male (Hildebrand, ’29). Sperm, within the female tract of the hen, are known to live and retain their fertility for two or three weeks or even longer (Dunn, ’27) . In the duck the duration of sperm survival is much shorter (Hammond and Asdell, ’26).

Among mammals, the female bat probably has the honor of retaining viable sperm in the genital tract for the longest period of time, for, while the female is in hibernation, sperm continue to live and retain their fertilizing power from the middle of autumn to early spring (Hartman, ’33; Wimsatt, ’44). According to Hill and O’Donoghue (T3) sperm can remain alive within the Fallopian tubes of the Australian native cat, Dasyurus viverrinus, for “at least two weeks.” However, it is problematical whether such sperm are capable of fertilizing the egg, for motility is not the only faculty necessary in the fertilization process. In most mammals, including the human female, sperm survival is probably not longer than 1 to 3 days. In the rabbit, sperm are in the female genital tract about 10 to 14 hours before fertilization normally occurs; they lose their ability to fertilize during the early part of the second day (Hammond and Asdell, ’26). In the genital tract of the female rat, sperm retain their motility during the first 17 hours but, when injected into the guinea pig uterus, they remain motile for only four and one-half hours. However, guinea-pig sperm will remain alive for at least 41 hours in the guineapig uterine horns and Fallopian tubes (Yochem, ’29).

E. Sperm Survival Outside the Male and Female Tracts

1. In Watery Solutions Under Spawning Conditions In watery solutions in which the natural spawning phenomena occur, the life of the sperm is of short duration. The sperm of the frog, Rana pipiem, may live for an hour or two, while the sperm of Fundulus heterocUtus probably live 10 minutes or a little longer. In some other teleost fishes, the fertilizing ability is retained only for a few seconds.


2. Sperm Survival Under Various Artificial Conditions; Practical Application in Animal Breeding


One of the main requisites for the survival of mammalian and bird sperm outside the male or female tract is a lowered temperature. The relatively high temperature of 45 to 50"^ C. injures and kills mammalian sperm while body temperatures are most favorable for motility of mammalian and bird sperm; lower temperatures reduce motility and prolong their life. Several workers have used temperatures of 0 to 2° C. to preserve the life of mammalian and fowl sperm, but a temperature of about 8 to 12*^ C. is now commonly used in keeping mammalian and fowl sperm for purposes of artificial insemination. Slow freezing is detrimental to sperm, but quick freezing in liquid nitrogen permits sperm survival even at a temperature of -—195° C. (See Shettles, ’40; Hoaglund and Pincus, ’42.)

Another requirement for sperm survival outside the genital tract of the male is an appropriate nutritive medium. Sperm ejaculates used in artificial insemination generally are diluted in a nutritive diluent. The following diluent (Perry and Bartlett, ’39) has been used extensively in inseminating dairy cattle:


Na 2 S 04 1.36 gr. )

Dextrose 1.20 gr. > per 100 ml. H 2 O.

Peptone 0.50 gr. )


Also, a glucose-saline diluent has been used with success (Hartman, ’39, p. 685). Its composition is as follows:


Glucose

Na2HP0ol2H.,0

NaCl

KH2PO,


30.9 gr. \

t n /• Pe 1000 ml. HjO. 2.0 gr. ( ^

0.1 gr. )



Some workers in artificial insemination use one type of diluent for ram sperm, another for stallion sperm, and still another for bull sperm, etc.

Artificial insemination of domestic animals and of the human female is extensively used at present. It is both an art and a science. In the hands of adequately prepared and understanding practitioners, it is highly successful. The best results have been obtained from semen used within the first 24 hours after collection, although cows in the Argentine have been inseminated with sperm sent from the United States seven days previously (Hartman, ’39, p. 685).

F. Transportation of the Egg from the Ovary to the Site of Fertilization

1. Definitions

The transportation of the egg from the ovary to the oviduct is described as external (peritoneal) migration of the egg, whereas transportation within the confines of the female reproductive tract constitutes Internal (oviducal) migration. It follows from the information given above that the site of fertilization determines the extent of egg migration. In those species where external fertilization of the egg is the habit, the egg must travel relatively long distances from the ovary to the watery medium outside the female body. On the other hand, in most species accustomed to internal fertilization, the latter occurs generally in the upper region of the oviduct. Of course, in special cases as in certain viviparous fishes, such as Gambusia affinis and Heterandria formosa, fertilization occurs within the follicle of the ovary and migration of the egg is not necessary. The other extreme of the latter condition is present in such forms as the pipefishes. In the latter instance the female transfers the eggs into the brood pouch of the male; here they are fertilized and the embryos undergo development (fig. 106).

2. Transportation of the Egg in those Forms Where Fertilization Occurs in the Anterior Portion of the Oviduct

a. Birds

A classical example of the activities involved in transportation of the egg from the ovary to the anterior part of the oviduct is to be found in the birds. In the hen the enlarged funnel-shaped mouth of the oviduct or infundibulum actually wraps itself around the discharged egg and engulfs it (fig. 31). Peristalsis of the oviduct definitely aids this engulfing process. Two quotations relative to the activities of the mouth of the oviduct during egg engulfment are presented below. The first is from Patterson, TO, p. 107:

Coste describes the infundibulum as actually embracing the ovum in its follicle at the time of ovulation, and the writer [i.e., Patterson] has been able to confirm his statement by several observations. If we examine the oviduct of a hen that is laying daily, some time before the deposition of the egg, it will be found to be inactive; but an examination shortly after laying reveals the fact that the oviduct is in a state of high excitability, with the infundibulum usually clasping an ovum in the follicle. In one case it was embracing a follicle containing a half-developed ovum, and with such tenacity that a considerable pull was necessary to disengage it. It seems certain, therefore, that the stimulus which sets off the mechanism for ovulation is not received until the time of laying, or shortly after.

If the egg falls into the ovarian pocket (i.e., the space formed around the ovary by the contiguous body organs ) , the infundibulum still is able to engulf the egg. Relative to the engulfment of an egg lying within the ovarian pocket, Romanoff and Romanoff, ’49, p. 215, states:

The infundibulum continues to advance, swallow, and retreat, partially engulfing the ovum, then releasing it. This activity may continue for half an hour before the ovum is entirely within the oviduct.

b. Mammals

In those mammals in which the ovary lies free and separated from the mouth of the oviduct (figs. 29, 111) it is probable that the infundibulum moves over and around the ovary intermittently during the ovulatory period. Also, the ovary itself changes position at the time when ovulation occurs, with the result that the ovary moves in and out of the infundibular opening of the uterine tube (Hartman, ’39, p. 664). In the Monotremata (prototherian mammals) during the breeding season, the enlarged membranous funnel (infundibulum) of the oviduct engulfs the ovary, and a thick mucous-like fluid lies in the area between the ovary and the funnel (Flynn and Hill, ’39). At ovulation the relatively large egg passes into this fluid and then into the Fallopian tube. In the rat and the mouse which have a relatively closed ovarian sac, the bursa ovarica, around the ovary (figs. 37, 112) contractions of the Fallopian tube similar to those of other mammals tend to move the fluid and contained eggs away from the ovary and into the tube. Thus it appears that the activities of the mouth and upper portions of the oviduct serve to move the egg from the ovarian surface into the reproductive duct at the time of ovulation in the mammal and bird. This method of transport probably is present also in reptiles and elasmobranch fishes. In the mammal this activity has been shown to be the greatest at the time of estrus. The estrogenic hormone, therefore, is directly involved in those processes which transport the egg from the ovary into the uterine tube.

In women, and as shown experimentally in other mammals, the removal of the ovary of one side and the ligation or removal of the Fallopian tube on the other side does not exclude pregnancy. In these cases, there is a transmigration of the egg from the ovary on one side across the peritoneal cavity to the opening of the Fallopian tube on the other where fertilization occurs. This transmigration is effected, presumably, by the activities of the intact infundibulum and Fallopian tube of the contralateral side.

Another aspect of egg transport in the mammal is the activity of the cilia lining the fimbriae, mouth, and to a great extent, the ampullary portions of the uterine (Fallopian) tube itself. The beating of these cilia tend to sweep small objects downward into the Fallopian tube. However, these activities are relatively uninfluential in comparison to the muscular activities of the infundibulum and other portions of the Fallopian tube.

Egg transport between the ovary and the oviduct is not always as efficient as the above descriptions may imply. For, under abnormal conditions the egg “may lose its way” and if fertilized, may begin its development withiir the spacious area of the peritoneal cavity. This sort of occurrence is called an ectopic pregnancy. In the hen, also, some eggs never reach the oviduct and are resorbed in the peritoneal cavity.

3. Transportation of the Egg in Those Species Where Fertilization is Effected in the Caudal Portion OF the Oviduct or in the External Medium

a. Frog

In the adult female of the frog (but not in the immature female or in the male) cilia are found upon the peritoneal lining cells of the body wall, the lateral aspect of the ovarian ligaments, the peritoneal wall of the pericardial cavity and upon the visceral peritoneum of the liver. Cilia are not found on the coelomic epithelium supporting and surrounding the digestive tract, nor are they found upon the epithelial covering of the ovary, kidney, lung, bladder, etc. (fig. 113). (See Rugh, ’35.) This ciliated area has been shown to be capable of transporting the eggs from the ovary anteriad to the opening of the oviduct on either side of the heart (fig. 113) (Rugh, ’35). In this form, therefore, ciliary action is the main propagating force which transports the egg (external migration) from the ovary to the oviduct. Internal migration of the egg (transportation of the egg within the oviduct) also is effected mainly by cilia in the common frog, although the lower third of the oviduct “is abundantly supplied with smooth muscle fibers,” and “shows some signs of peristalsis” (Rugh, ’35). The passage downward through the oviduct to the uterus consumes about two hours at 22 C. and, during this transit, the jelly coats are deposited around the vitelline membrane. The jelly forming “the innermost layer” is deposited “in the upper third of the oviduct, and the outermost layer just above the region of the uterus.” The ciliated epithelium, due to the spiral arrangement of the glandular cells along the oviduct, rotates the egg in a spiral manner as it is propelled posteriad (Rugh, ’35). Once within the uterus, the eggs are stored for various periods of time, depending upon the temperature. During amplexus, contractions of the uterine wall together, possibly, with contractions of the musculature of the abdominal wall, expel the eggs to the outside. At the same time, the male frog, as in the toad, discharges sperm into the water over the eggs (fig. 103). In the toad, the eggs pass continuously through the oviduct and are not retained in the uterus as in the frog (Noble, ’31, p. 282).


b. Other Amphibia

The transport of the eggs to the site of fertilization in other anuran amphibia presumably is much the same as in the frog, although variations in detail may occur. In the urodeles, however, conditions appear to diverge from the frog pattern considerably. As mentioned previously, fertilization of the eggs of Salamandra atra may occur within the peritoneal cavity before the egg reaches the oviduct, while fertilization in most urodeles occurs internally in the oviduct, either posteriorly or in some cases more anteriorly. In this amphibian group, the ostium of the oviduct is funnel-shaped and is open, whereas in the frog it is maintained in a constricted condition and opens momentarily as the egg passes through it into the oviduct. (Compare figs. 34, 113.) The open condition of the oviducal ostium in the urodeles suggests that the ostium and anterior part of the oviduct may function as a muscular organ in a manner similar to that of birds and mammals.

c. Fishes

Egg transport in the fishes presents a heterogeneous group of procedures. In the cyclostomes the eggs are shed into the peritoneal cavity and are transported caudally on either side of the cloaca to lateral openings of the urogenital sinus. The eggs pass through these openings into the sinus and through the urogenital papilla to the outside. Contractions of the musculature of the abdominal wall may aid egg transport.

In most teleost fishes, the contraction of ovarian tissue together with probable contractions of the short oviduct is sufficient to expel the eggs to the outside (fig. 28). A somewhat similar condition is found in the bony ganoid fish, Lepisosteus, where the ovary and oviduct are continuous. However, in the closely related bony ganoid, Amia, the eggs are shed into the peritoneal cavity and make their way into an elongated oviduct with a wide funnelshaped anterior opening and from thence to the outside. A similar condition is found in the cartilaginous ganoid, Acipenser. In the latter two forms, the anatomy of the reproductive ducts in relation to the ovaries suggests that the egg-transport method from the ovary to the ostium of the duct is similar to that found in birds and mammals. Muscular contractions of the oviduct probably propel the egg to the outside where fertilization occurs. This may be true also of the salmon group of fishes, including the trout, where a short, open-mouthed oviduct is present. In the lungfishes (Dipnoi) the anatomy of the female reproductive organs closely simulates that of urodele amphibia. It is probable that egg transport in this group is similar to that of the urodeles, although fertilization in the Dipnoi is external.


G. Summary of the Characteristics of Various Mature Chordate Eggs Together with the Site of Fertilization and Place of Sperm Entrance into the Egg

Gallus {domesti- 31 mm. vertical. Zona radiata or vitelline Strongly telolecithal Infundibular re- Disc of proto cus) gallus (hen) 32 mm. trans- membrane before egg gion of oviduct; plasm at ani verse, 34 mm. leaves ovary. Envelopes of possibly also in mal pole

product of the egg and follicle cells; and (c) as a secretion product of the follicle cells. The last theory probably is the true


Didelphys 12Q-140 jti Zona pellucida; albuminous Isolecithal with large yolk Infundibular re- Probably at nu virginiana and outer chitinous layer spherules gion of Fallo- clear pole of (opossum) laid down in Fallopian tube pian tube egg


rovided wi cida when ovary; als( rona cells


ona plus c buminous in Fallop rona radi sipated


alopus aquat (mole)

US musculus (mouse)

attus rattus (rat)

avia porcellu (guinea pig^

ryctolagus

cunkulus

(rabbit)

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