The mammalian egg

Abstract

Primary oocytes exist in large numbers in the ovarian cortex of young animals. They themselves seem incapable of division and their abundance is owing to the earlier multiplication of the oogonia from which they have differentiated. As a feature of differentiation, the oocyte nucleus starts upon the early prophase changes of the first meiotic division, the chromosomes become somewhat condensed, and the nucleus then passes into the dictyate stage. Those oocytes that are not destined, as many are, for early degeneration remain in this stage until meiosis is suddenly resumed shortly before or soon after ovulation. The precise form assumed by the chromosomes in the dictyate stage is uncertain though they clearly lose their earlier partially condensed appearance. In oocyte nuclei in fish, amphibians, reptiles and birds, the chromosomes take on the form of fine long threads bearing numerous lateral loops, and are referred to as lampbrush chromosomes. Their special significance is still conjectural-they may play a part in yolk synthesis. Equivalent structures have yet to be demonstrated in mammalian oocytes. Recent observations of Ohno, Kaplan and Kinosita (i960) showed that the two X chromosomes in rat oocytes are isopycnotic, both at the first meiotic prophase and the second meiotic metaphase. This is in contrast to the positively heteropycnotic state of the XY bivalent in spermatocytes, as previously demonstrated by these workers (Ohno, Kaplan and Kinosita, 1957, 1958), and they suggest that the condition in spermatocytes represents an evolved mechanism that prevents crossing-over and ensures isolation of the femaledetermining chromosome from the male-determining chromosome. Crossing-over between the two X chromosomes in oocytes, on the

Fertilization in Vitro of Mouse and Hamster Eggs After the Removal of Follicular Cells

“…It has been postulated that the presence of cumulus oophorus may decrease the incidence of polyspermy and increase the chance of fertilization, and that the corona radiata cells may play a rôle in the orientation of spermatozoa approaching the eggs (Austin, 1961).…”

“…In the rabbit, the cumulus oophorus is said to be dispersed by hyaluronidase released from the sperm acrosome (Austin, 1961) and corona radiata cells are removed by a tubai factor (Swyer, 1947). It has been postulated that the presence of cumulus oophorus may decrease the incidence of polyspermy and increase the chance of fertilization, and that the corona radiata cells may play a rôle in the orientation of spermatozoa approaching the eggs (Austin, 1961).…”

Species Differences in the Lectin-Binding Sites on the Zona Pellucida of Rodent Eggs

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Mammalian eggs are surrounded by a moderately thick transparent coat, the zona pellucida, which is believed to be an important site of sperm recognition and of the block to polyspermy (Braden, Austin & David, 1954;Austin, 1961;Dickmann, 1962;Yanagimachi, 1964;Barros, 1968

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Determinism and Inevitability

“…He considers, for instance, Austin's famous question whether he could have holed a short putt which in fact he missed. 10 In considering this question, says Dennett, we need to choose the set X of possible worlds which we are to canvass in order to see whether Austin could have holed the putt. If we use the narrow method described above, then we will choose the set X of possible worlds which are identical to the actual world at some time t 0 immediately prior to the putt, and if determinism is true, this set of worlds has only one member, the world in which Austin misses.…”