Volume 4

A reference handbook of the medical sciences : embracing the entire range of scientific and practical medicine and allied science / by various writers ; edited by Albert H. Buck.

Date:: 1888-1895

Credit: A reference handbook of the medical sciences : embracing the entire range of scientific and practical medicine and allied science / by various writers ; edited by Albert H. Buck. Source: Wellcome Collection.

18/838 (page 6)

$polar globule from the ovum, while in the ovary. One spermatozoon gets into the yolk proper, and its entrance apparently prevents the penetration of other spermatozoa —how is undetermined. The tail of the spermatozoon soon disappears ; while the head enlarges, probably by the imbibition of fluid from the surrounding yolk, and thus becomes a nucleus-like body, the male pronucleus. The passage of the spermatozoa through the zona was first discovered by Martin Barry, in 1843, and although his statement was received with considerable hesitation by his contemporaries, it has since had competent con- firmation repeatedly. Warneck20 is said to have been the first (1850) to see the two pronuclei, but their signifi- cance was not perceived. The nature of the male pro- nucleus was first recognized by Oskar Hertwig,10 who traced its genesis in the ova of echinoderms from the spermatozoon. The fact that the male pronucleus is the metamorphosed spermatozoon has since been confirmed by Selenka,'6 Ed. van Benedeu,19 Nussbaum,13 Eberth,4 Platner,14 and others. Although a number of the spermatozoa make their way into the perivitelline space, probably always one alone normally enters the yolk to there form a pronu- cleus. The best observers are agreed upon this point, and in all species, the observations upon which have cov- ered the whole series of steps in the impregnation, there has been found in normal cases always a single male pro- nucleus. Schneider's statements to the contrary have been definitely corrected. Bambeke,25 Kupffer,7 and Kupffer and fienecke 8 have observed that several sper- matozoa actually enter the yolk in batrachians and petro- myzon. Hertwig, however, found only one male pro- nucleus in the frog, and there has as yet been no evidence adduced that several spermatozoa are concerned in the final phases of impregnation. Fol observed that star-fish eggs are normally impregnated by one spermatozoon ; but if they are exposed to the action of carbonic acid they may, while so poisoned, be impregnated by several spermatozoa, but the subsequent development in this case is abnormal; apparently each pronucleus becomes a separate centre of development. The term micropyle is applied to various structures, and does not in all cases designate a passage through Flo. 1811.—Anterior Pole of the Orum of the Petromyzon, with a Sperma- tozoon^)., entering the Micropyle (mi). (After Calberla.) p.v., Perivi- telline space; z, zona pellucida ; a, pathway to female pronucleus, n ; yk, yolk. which spermatozoa enter—thus the micropyle of holo- thurians is merely a structure left iu the envelopes in consequence of the way the egg is developed. A micro- pyle has been observed in the ova of teleosts and the lamprey, but it is doubtful whether it is, like the micro- pyles in the hard-shelled eggs of insects, intended for the passage of spermatozoa. Ca'berla2 saw the micropyle thus used in the lamprey, and it is possible that the fruc- tifying spermatozoon enters that way, although Kupffer and Benecke 8 state that spermatozoa may also pass di- rectly through the zona. A good synopsis of the obser- vations on the lamprey is given by Hensen (Hermann's Handbuch, VI., ii., 120). We have only to notice that there is a special band of non-granular protoplasm lead- ing from the micropyle to the egg nucleus (Fig. 1811), and serving as a path for the spermatozoon ; a similar band has been observed in amphibian eggs by Hertwig and Bambeke ; it serves perhaps as a mechanism to guide the male to the female pronucleus. The manner in which additional spermatozoa are ex- cluded, 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 sperma- tozoon 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- velopes may be pierced at any point, as in mammals, there must of course be some other device. Fol has maintained that this is found in the starfish 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. Gentralbl., 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 off and are knocked about by the spermatozoa in the perivitelline space. In this case there seems to be a portal opened just long enough for one spermatozoon to enter. As the phenomena to be explained is common to all ova, its causa- tion is presumably fundamentally identical in all cases. Beyond this surmise our present knowledge does not per- mit us to go. The hypothesis may be suggested that the attractive power of the ovum is annulled or weakened by the formation of the male pronucleus. It is probable that the tail of the spermatozoon, when that appendage exists, disappears within the yolk. In a land-snail (arion), Platner 11 has traced this process very clearly. Only a portion of the tail enters the yolk ; but the part within acquires the prop- erty of staining readily, and so may easily be observed. He reports that the head and tail separate; the head only conjugates with the female pronucleus, while the tail still remains distinct even after segmentation has been initiated (Fig. 1815, A). The disappearance spermatozoon (s), and a 0f the tail has been recorded by Protuberance of the Yolk mQ^ observer, As Hertwig says Fig. 1812.—Egg of Toxop- ncustes, with entering rising toward it, Selenka.) (After (loc. cit.,n p. 23), all these careful observations yield the assured con- clusion 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 for- mation of a slight protuberance on the surface of the yolk, rising up toward the spermatozoon. This protu- berance (Fig. 1812) may remain, as in echinoderms, until the spermatozoon meets it, and by penetrating it enters the ovum ; or it may retract before the spermatozoon passes through the envelopes, and even withdraw, as in petro- myzon, so far from the advancing spermatozoon as to change into a depression (Fig. 1811). The protuberance lasts only a few minutes. In bufo, according to Kupf- fer,1 several spermatozoa enter the yolk and a protuber- ance rises toward each one, as if the yolk were actively striving to reach the male element. The relative size of the two pronuclei varies consider- ably in different species, and is probably a secondary and unimportant matter. Each pronucleus, when it first ap- pears, 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 the maturation of the ovum. If it enters early, as in Umax (Mark), the male pronucleus enlarges equally with the female (compare Fig. 1813), but if'late, as in the allied arion (Platner), then it appears (Fig. 1815) considerably smaller than the al- ready swollen female pronucleus. Oskar Hertwig, in his third paper on maturation,' p. 171, first gave this expla- nation, and pointed out that in the starfish (asterias), if the$