Paper - Studies in the development of the opossum 5 (1920)
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Hartman CG. Studies in the development of the opossum Didelphys virginia L. V. The phenomena of parturition. (1920) Anat. Rec. 19(5): 251-262.
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Studies in the Development of the Opossum Didelpbys Virginiana L.
V. The Phenomena of Parturition
Carl G. Hartman
The University of Texas, School of Zoology
- Parts 1 and 2 (History of the early cleavage — Formation of the blastocyst) appeared in the Journal of Morphology, volume 27, number 1, March, 1916; and Parts 3 and 4 (Description of new material on maturation, cleavage and entoderm formation — The bilaminar blastocyst) appeared in the Journal of Morphology, volume 32, number 1, March. 1919. These four parts may be obtained from the publishers. Contributions from the School of Zoology, the University of Texas, no. 143.
The Method of Transfer of Young to the Pouch
So far as the writer has been able to discover, there exists in the nterature only one account of the actual birth of any marsupial, notwithstanding the abundance of opossums in America and the variety of all marsupial fauna in Australia. Nor does that foremost of all students of marsupial embryology, Prof. J. P. Hill, refer to this topic, although on one occasion ('00, p. 371) he killed a specimen of Dasyurus after "one only of the young had been born." ^leigs ('47) and Selenka ('87) examined pouch young of the opossum immediately after birth, but made no observation on parturition.
The single recorded observation referred to is that of Dr. Middleton Michel, of South Carolina ('50), who, on January 28, 1847, witnessed the copulation of a pair of opossums, and fourteen days and seventeen hours later saw the birth of the foetuses. In order to show, however, that Doctor ^Michel failed to see the actual passage of the 3'oung to the pouch, two essential paragraphs on the point at issue are here quoted:
The pregnant female was found standing on her hind legs; her body was much bent, and propped up against the rornor of the cage; her muzzle in immediate contact with the cloacal opening, which was red, tiunefied and distended: a young ap|)eared at the opening, and was conveyed by the mother's mouth to the pouch, or perhaps was rather licked in, as her tongue seemed busily employed within, around and about the pouch.
The young are expelled first into the vaginal cul-<le-sac, in which they remain for a short time, on the contraction of which they are forced along the vaginal canals one by one; parturition is thus ver>' much prolonged, owing to the circuitous route which the young are obliged to take, and the delay therein- occasioned between the birth of each is the object of the peculiar modification of these parts in this animal, as it affords the requisite time emjiloyed in the convej'ance of the young to the pouch and their adaptation to the teat.
It is quite clear from the language of this quotation that Doctor Michel did not actuallj- witness the migration of the embryos and that he merely guessed at the method emploj-ed by the mother, since he was not sure whether she used her mouth or her tongue. It will, moreover, be shown below that Doctor Michel was also mistaken in presuming that the young at birth pass out by way of the lateral vaginal canals. The observations recorded below indicate that the marsupial female does not actually transfer the foetuses to the pouch and that Doctor Michel's interpretation, as well as the prevailing notion in accordance therewith, is not borne out by the facts.
Some preliminary observations
A series of observations and experiments during the last four or five breeding seasons of the opossum had enforced the conviction that the young reach the pouch and find the teat by their own efforts and are not placed on the teats by the mother's tongue or lips, ^^h^• should it be necessary, one may ask, in the absence of actual observation, to presume such undue skill and sensitivit}- in the adult when a pure instinctive reaction on the part of the young will suffice?
On several occasions I experimented with newly born pouch young, gently removing them from the teats to which they had firmly attached themselves by means of their powerful tongues.
I quote from my notes in one case (no. 301, experimented upon in the presence of Dr. C. H. Heuser, of The Wistar Institute, January 20, 1917) :
Female tied down and pouch opened. Young which were removed from teats crawled about, moving hands alternately, as in swimming. Were able to crawl among hairs and find teats by their own efforts. One specimen, removed three times, found teat each time and three others found teats after wandering about.
These experiments certainly argued strongly in favor of some little independence of action on the part of these 'embryos,' a term that Doctor Meigs ('47) would have us abandon when speaking of these breathing, sanguiniferous, digesting pouch young."
On February 6, 1917, on opening specimen no. 402 under anesthesia, I was surprised to find a collapsed but very vascular uterus, as if birth had just taken place. This proved to be the case, for on remo\'ing the animal from the table I found that the entire litter of foetuses had been expelled during the operation. They were mostlj' still ali\'e, entangled in foetal envelopes and immersed in the foetal fluid. To some of the foetuses the umbilicus was still attached ; others were free, but no navel could be seen in any case. None of the foetuses, even after being freed of membranes and liquids, could crawl about, as they were apparently drowned in their own embryonic fluid. It seemed likely, therefore, that the emlsryos, on emerging from the vagina, need the assistance of the mother to lick away the fluid expelled from them, and this w-as later verified by actual observation.
Embryos near term were also removed from the uterus, freed of their envelopes, and allowed to crawl about over the mother, which they did for at least fifteen minutes.
On one occasion I removed one uterus three days before term (no. 131), and about the time that birth was to be expected from the remaining uterus I injected, some pituitrin subcutaneously, hoping to witness parturition thus brought on. But owing to the fact that abortion had pre\dously taken place, as was afterward learned, onlj' mucus was extruded from the genital orifice.
It is intorpstinp; to note, however, that after the injection of pituitrin the female Uctced out the pouch at freciuent intervals, an act which probably always precedes parturition.
The birlh of the opossum
Specimen no. 443 was broupiht to the laboratory February 2, 1920, having been captured uninjured several nights before. She was a healthy female of medium size, and In' palpation of the mammary glands, after the method which I have described at another place ('19, p. 24), T recognized her as pregnant and Hkely to give birth within several days. I removed her to my home, where she was kept under observation night and day, and the success which attended the undertaking is largely due to my wife's enthusiasm and perseverance.
The animal was i)laced just outside a window in a cage illuminated within by a red electric light, which arrangement was least disturbing to the animal as she was insulated against noises from within the room; the sight of persons moving about in the room caused little response on the part of the animal, but slight noises near the cage startled her great h'.
At 10:30 P.M., February (>, 1920, the animal showed signs of restlessness and soon began cleaning out the pouch, which she did about four times. Then began a short series of spasmodic contractions of the abdominal wall, after which she came to a sitting posture with legs extended. .\t no time did she stand on her hind legs, as Doctor Michel seems to have observed, for such
t po.sition is certainly strained and unnatural. I once had an opossum give birth while she was confined in a Inulaji sack in which she was carried to the laboratory. In this case it was assuredly' impo.s-sible for her to stand on her hind legs during parturition.
After assuming the sitting i)osture, our specimen bent her body forward and licked the vulva: however, her position at this time was such that we could not see the embryos, which very likely passed into the pouch with the fust licking of the genital opening. Hence we went to the outside where we could plainly hear her lap up the chorionic fluid; then suddenly a tinj- bit of flesh appeared at the vulva and scampered up over the entanglement of hair into the pouch to join the other foetuses, which now could be seen to have made the trip without our having observed them. Unerringlj' the embryo tra^'eled by its own efforts; without any assistance on the mother's part, other than to free it of liquid on its first emergence into the world, this ten-day-old embryo, in appearance more like a worm than a mammal, is able, immediately upon release from its liquid medium, to crawl a full three inches over a difficult terrain. Indeed, it can do more: after it has arrived at the pouch it is able to find the nipple amid a forest of hair. This it must find — or perish.
Having now satisfied ourselves as to the manner in which the 3'oung opossum reaches the pouch, we etherized the female, hoping still to find some of the embryos within the genital tract. But it happened that we had witnessed the last of the litter make the journey. The pouch contained a squirming mass of eighteen red embryos of which twelve were attached, though thirteen might have been accommodated. The remainder were, of course, doomed to starvation. Even some of these unfortunates, however, held on with their mouths to a flap of skin or to the tip of a minute tail, while several continued to move about.
With the mother under the influence of ether, we now gently pulled off a number of embrj'os from the teats in order to test their reactions. The teats had ah-eady been drawn out from about a millimeter in height to double that length, doubtless bj' the traction of the embryo itself, for the bottom of the pouch certainly presented a busy scene with each member of the closepressed litter engaged in verj^ active breathing and sucking movements.
One detached young, placed near the vulva, crawled readily back into the pouch. Two or three others regained the teats after some delaj^, and one wanderer, which lost out in the first scramble, found a vacated teat and attached itself even after twenty minutes' delay, showing that the instinct to find the teat . persists for some time. If the skin be tilted, the embryos, can be made to travel upward and even away from the pouch, for they are negatively geotropic.
For locomotion the embryo employs a kind of 'overhand stroke,' as if swimming, the head swajnng as far as possible to the side opposite the hand which is taking the jiropcUing stroke. With each turn of the head the snout is touched to the mother's skin as if to test it out, and if the teat is touched, the embryo stops and at once takes hold.
It is thus ajiparent that the opossum embryo at birth possesses not only fairly well-developed respiratory and digestive systems, but that it has attained a neuromuscular development sufficient to enable it to find its place in the pouch where food and shelter await it.
The Number of pouch young
Most female opossums possess thirteen teats, of which usualljonlj' the posterior eleven are functional. I have often found as many as eleven pouch yovmg attached, but only in two cases as many as twelve. Doctor ^leigs ('47) on one occasion found thirteen. I have seen litters consisting of fifteen, seventeen, and eighteen newlj- born j'oimg in the pouch, with as few as seven attached to teats, and have removed from pregnant uteri as many as twenty-two normal foetuses near term. Such overproduction with consequent mortality has already been pointed out for the opossum and other marsupials (Hill, '10, '11: Hartman, '19).
In the popular mind the generation of no animal is so shrouded in mystery as that of the opossum. From New Jersey to Texas several beliefs are current which it might be well to state at this point.
There is a wide-spread notion that copulation takes place in the nostril of the female and that the 'fruit of conception' is blown into the pouch. This superstition rests upon two observed facts: first, that the opossum penis is dichotomous and, second, that the female licks out the pouch immediately prior to parturition.
Another notion is that the pouch young is organically connected with, or 'grown to,' the teat, in fact so intimatelj^ that bleeding results from the forced separation of the pouch young. Doctor Meigs ('47) already showed that this is not the case.
Doctor ]Meigs mentions and refutes the idea prevailing in his time that the pouch young produces a teat wherever it happens to take hold of the skin in the pouch.
Finally, it is often stated that the marsupial mother pumps milk into the pouch young. Whether or not this is true the writer does not know, but certain it is that from the very beginning the young opossum engages in active sucking movements.
The Passage of the Foetuses from the Uterus
As is, of course, well known, the opossum, as a member of the order Marsupalia, possesses two uteri. These do not communicate posteriorly, but open each into a separate shallow cul de sac, on either side of a median partition. Each cul de sac communicates laterallj' with a loop, the 'lateral vaginal canal' (Hill, '97), which cur^-es laterad, then caudad and mediad, until near the midline the two canals almost touch; and from this point backward they Ue parallel until they empty into the 'median vaginal canal' (Hill, '97) or urogenital passage (Owen, '68). The lateral vaginal canals thus resemble two question marks placed face to face; the curved portions lie in the body cavity, the 'stems' are imbedded in the connective tissue of the urogenital strand. The urethra forms a third parallel tube, lying in the midline ventrad to the straight portion of the lateral A-aginal canals and emptying with them into the median vaginal canal.
In two Austrahan species Hill (Parameles, Dasj-urus; Hill, '98, '00) made the surprising discovery that the embryos at birth do not pass out through the lateral vaginal canals, but break through by a cleft-like rupture, the 'pseudo vaginal canal, directly into the median vaginal canal from the culdesac into which the os uteri opens. The new passage is described as a split in the connective tissue, at no time lined with epithelium and containing fragments of foetal membranes together with leucocvtes and maternal blood clots.
I have on several occasions demonstrated in the opossum the existence of the pseudovaginal passage discovered by Hill. In sjiecinien no. 402. aheady iiiontioiiod as aborting under an abdominal operation, one could follow a bloody trail direct into the median vaginal canal exactly as Hill had described it The hemorrhage was less severe in no. 443, the birth of whose young has been described above, but the new passage was ea.sil}- demonstrable. The organs were fixed in Bouin's fluid and sectioned. The findings are quite in accord with those of Hill. The pseudovaginal canal is seen to be simply a slit in connective tissue between the bladder and urethra voiitrally and the caudal ends of the lateral vaginal canals dorsally. In formaldehyde preparations of the organs taken from non-pregnant females such a pseudovaginal passage can with great ease be pushed through; that is, the urethra maj- very readily be separated from the parts dorsal to it. It appears quite certain that the contraction of the alxlominal and the uterine walls is sufficient to force the new passage at the tune of birth.
The embryonic envelopes are partly retained within the uterus, a fact already noted by Osborn ('87) for the opossum, and partly scattered along the median vaginal canal. None were found in tiie lateral vaginal canals either by Osborn or by the writer. It is possible that an embryo may even drag parts or all of its foetal membranes to the exterior, in which case the mother may lick it free; but my only evidence on this point is the presence of the foetal membranes about many of the embryos in the case of one abortion.
The opossum should therefore lie added to the list of marsupials which force the 'pseudovaginal canal' at parturition.
One might suppose from this that the lateral vaginal canals would pos.sess a special function. The writer believes with Hill that the}' function as rcceptacula scminis, .since in the marsupials several days elapse between copulation and ovulation. In the opossum the enlargement of the canal is one of the striking features of the prooestrus period. .\t oestrus they "have attained an enormous size and are filled to turgidity with a thin, lymi)li-like fluid. Soon after ovulation thej' shrink almost to the resting stage and are filled with cheesy masses of cpithehal cells, which remind one of a similar phenomenon described bj^ Stockard and Papanicolaou ('17) for the guinea-pig at oestrus.
Several months after the foregoing paper had been received by the editor of this journal the writer received a note from Dr. H. H. Donaldson, of The Wistar Institute, in which he stated that he had learned from Dr. N. Hollister, Superintendent of the National Zoological Park, Washington, D. C, of a pubhshed account of parturition in Macropus rufus, the deer kangaroo. The article in question is in the nature of a communicatioii by the observer, Mr. A. Goerling, to the 'Western Mail,' of Perth, Australia, and was published January 3, 1913. Doctor Hollister's kindness in having the article copied makes it possible to present this interesting account to the readers of The Anatomical Record and thus render it more generally available to zoologists. The accounts of the birth of Didelphys virgiiiiana, as detailed above, and of INIacropus rufus, as reported by ^Ir. Goerling, are seen to be in perfect agreement on the one essential point, namel}^ that the young reach the pouch and find the teat by their own efforts and entirely without the assistance of the mother. It would seem, therefore, that this will be found to hold universally among the numerous species of the ]Marsupialia. The following are Mr. Goerling's notes dated December 19, 1912:
The Birth of the Kangaroo
The question of how the young kangaroo comes into the pouch has long been looked upon as answered. According to observations made, the young is born and placed on the pap by its mother, and this view has been accepted In" zoologists.
On the 25th of February, 1906, 1 had the good fortune to make the most interesting and astounding obseivation. I had a number of Macropus rufus and M. cervinus in my possession, caged in varioussized cages. On the morning of the above mentioned date I was attracted by the peculiar behavior of a female M. rufus. She refused the feed placed before her; and on seeing blood marks in the cage, I came to the cont'lusic^n tliat the nniinal liad just given birth to a young one. Shi' was sittiinj in tliat resting ]iosition in which kangaroos can often be seen. Tlie tail pas.<eii forward through the legs, thus she was sitting almost entirely on the thick part of her tail. 8hc took no notice of my presence, although not more than three weeks in captivity, and was busy licking and cleaning herself. Presently she lifted her head, when I was astonished to see a young kangaroo clinging to the long fur about four inches below the opening of the pouch.
'The italics are mine.
It moved about slowly, ven- slowly, through the fur upwards, using the arms in its progress, and conlittiuiUy moving the head from side to side, thus a.ssi.sting the upward movement. Nearly 30 minutes were required by the little wanderer to reach the top of the pouch, the last end in a semicircle. During the whole of this time the mother paid no attention to her offspring, offering no ass-istance, ami leariny it entirely to its own exertions. She then became restless; and not wishing to disturb her, I moved a short distance away, when she at once started to feed. A little later I paid another visit to her cage. She was sitting upright, the young one had disajjpeared, but the fur was still bearing evidence of the struggle, a plain visible track leading to and ending on the top of the pouch.
Now I had the explanation of a previous observation, but which I misconstrued at the time. I had a female Macropus woodwardi — Woodward's kangaroo — in captivity; and noticing blood stains in the cage. I beheved the animal was hurt. I then noticed just such a young kangaroo clinging to the fur below the pouch, and thought the mother by restless movements had dislodged it.
My observation of the 25th of Februaiy, 1006, proves that the new born kangaroo has to look after its own safety and reach the pouch mthotd the mother's as.vstance.
The arms of the new born kangaroo are strongly developed, the small hands open and close like a cat's paw, and by these strong little anns and hands the young one is enabled to labour its way to the pouch, the place of safety and nourishment.
The question now presents itself, how can the young, with such a hard and firmly closed mouth, attach itself to the pap? I am convinced that at the time of birth the mouth has a wider opening and is perhaps more elastic than such specimens possess which are found in the pouch of the mother. Once a young kangaroo is removed from the pap, it is unable to reattach itself.
As concluding proof that all newly born marsupials must reach the pouch by their own exertions, I mention that bandicoots, native cats and those very smallest of marsupials, the i)0uched mice, have the opening of the pouch in a reversed po.sition to the kangaroos and phalangers. I had once in my pos-scssion a ver>' small specimen of pouched mouse, having ten young ones in the pouch, each one not lugger than a grain of wheat. (Jnly through the opening of the pouch being reversed are these smallest of both mammals enabled to reach it with saftitv and without much exertion.
Hartman, Carl G. 1919 Studies in the development of the opossum (Didelphys virginiana L.)- Parts III and IV. Jour. Morph., vol. 32, no. 1, pp". 1-UO.
Hill, J. P. 1895 Preliminary note on the occurrence of a placental connection in Parameles obesula, and on the foetal membranes of certain macropods. Proc. Linn. Soc, New South Wales, vol. 10 (2nd ser.), part 4.
1897 The placentation of parameles (Contributions to the embryology of the Marsupialia I). Quart Jour. Micr. Sci., vol. 40, pp. .385-142.
1899. 1900 Contributions to the morphology and development of the female urogenital organs in the Marsupialia, no. 1. On the female urogenital organs in Parameles, with an account of the phenomena of parturition. Proc. Linn. Soc. N. S. Wales, vol. 24, pp. 42-82. Part I, March 29; nos. 2-5, id., vol. 25, pp. 519-532.
1900 Contributions to the embryology of the Marsupialia. Quart. Jour. Micr. Sci., vol. 43, pp. 1-22.
1900 On the foetal membranes, placentation and parturition of the native cat (Dasyurus viverrinus). Anat. Anz., Bd. 18, s. 364—373.
Hill and O'Donoghue 1913 The reproductive cycle in the marsupial Dasyurus viverrinus. Quart. Jour. Micr. Sci., vol. 59.
Meigs, Dr. Charles D. 1847 Reproduction of Didelphys virginiana. Proc. Am. Philosophical Soc, Philadelphia, vol. 4, pp. 327-330.
Michel, Dr. Middleton 1850 Researches on the generation and development of the opossum. Proc. Am. Assn. Adv. Sci., vol. 3, Charleston, S. C.
OsBORN, H. F. 1888 The foetal membranes of the marsupials: the yolk sac placenta in Didelphys. Jour. ^lorph., vol. 1, pp. 373-382.
Owen, Richard 1868 Anatomy of vertebrates, vol. 1, p. 682.
Selenka, E. 1887 Studien ueber Entwioklungsgeschiehte der Thiere. IV (1 and 2), Das Opossum (Didelphys virginiana). Wiesbaden.
Stockard, Charles R., and PAPANicoLAor, George N. 1917 The existence of a typical oestrous cycle in the guinea-pig with a study of its histological and physiological changes. Am. Jour. Anat., vol. 22, pp. 225265.
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