But the impregnated ovurn, instead of en tering the uterine cavity, may be accidentally detained in the tube, and undergo further de, vclopment there. The extent to which this development may proceed will depend in a great measure upon the capability of expansion of the tube walls ; a circumstance which seems to vary greatly in different individuals, and also in some degree according to the portion of tube which the ovum occupies.
The differences observable in this latter respect have led to a division of cases of tubal gestation into three varieties, viz. tubo-ovarian, tubal, and interstitial.
In the first variety, graviditas tu.bo-ovaria, the ovurn becomes developed in a sac, of which a principal portion appears to be fur nished by the hypertrophted walls of the infundibular end of the tube, and the proper tissue of the ovary combined. In the second, graviditas tubaria, the developed ovum occu pies some part of the canal of the free portion of the oviduct ; while in the third, graviditas interstitialis, the seat of development of the ovum is that part of the tube which traverses the uterine walls.
In the first, or tubo-ovarian variety, the parts supplying the principal foundation of the cyst, which surrounds the foetus, are in the first instance probably chiefly normal struc tures ; and it is easy to understand how, during the progress of growth of the ovutn, when the limit of expansibility of these parts has been reached, there may be superadded to them materials for the extension and fur ther growth of the cyst walls ; and in this way are apparently formed these large sacs, or artificial uteri, which have been sometimes observed to surround a fully developed foetus, and which in the course of their growth have come to include omentum, mesentery, or in testine, and other portions of the abdominal viscera or parietes, by which the sides of the sae become strengthened and enlarged.
As in the case of ovarian gestation for merly described, so in the varieties termed ovario-tubal, it is only when death has taken place during the early stages of formation of these embryo-bearing cysts that the exact nature and relation of the parts originally composing them can be made out. Hence the difficulty of determining, in more advanced stages of gestation, when other parts have been superadded, in what precise situation the development of the ovum was commenced.
And hence the probability that some at least of those cases which have been recorded from time to time as examples of the foetus deve loped in the cavity of the abdomen, and among the intestines (graviditas abdominally), may have been originally cases of the tubo-ovarian variety, in which the cyst walls, commencing their formation by the artificial union of the expanded termination of the oviduct with a portion of the ovarian parietes, have in the course of their growth come to include many other parts.
The second variety, which includes all cases strictly terrned tubal (graviditas tubaria), con stitutes by far the most common of all the forms of extra-uterine gestation. Here the ovum is developed within some part of the free portion of the tube, whose walls appear, from the examples which most of our mu seums furnish, to be capable of a very limited degree of expansion in most individuals. IIence, when the ovum has attained to a cer tain size, and usually by the time that the second or third month of gestation has been reached, rupture of the tubal wall occurs, followed by rapid death from hmmorrhage. And thus the parts are usually obtained for examination in such a state as to leave no room for question regarding the precise seat which the ovum occupies, and the nature of the parts enclosing it. For in these cases of early rupture the tube has contracted no adhesions with surrounding parts, and the walls of the embryo-bearing sac are formed of the parietes of the oviduct alone.
The third variety of tubal gestation, distin guished by M. Breschet under the title of Graviditas in uteri substantia; and by Profes sor Mayer, of Bonn, as Graviditas interstitialis, has been made known, particularly by an essay of the former devoted to this subject.* This variety differs from the last mentioned cljiefly in the circumstance, that the seat of development of the ovum is that portion of the canal of the tube which passes through the solid walls of the uterus. Here the sac surrounding the foetus is formed in a great measure at the expense of the proper uterine tissues, and consequently the parietes of these cysts exhibit under the microscope a very different composition from that which the tube walls show in the second variety.