Z8600021: /* Chapter III Fertilisation */

2019-12-31T04:20:25Z

Chapter III Fertilisation

Z8600021: /* Chapter III Fertilisation */

2019-11-23T02:27:25Z

Chapter III Fertilisation

← Older revision		Revision as of 13:27, 23 November 2019
Line 5:		Line 5:
	=Chapter III Fertilisation=		=Chapter III Fertilisation=

	The ~~Moment~~ of fertilisation is the conventional point of origin from which to date the existence of a new individual. We have seen in the last chapter that in fact many processes which are most important for the developing embryo occur before fertilisation, during the maturation of the egg. It cannot be denied, however, that fertilisation is, normally at least, the most crucial event within the continuous series of changes by which the new creature comes into being. It is not a simple occurrence, at which there is only one happening of importance; but its two important phases succeed one another quite quickly, and although they can be dissociated from one another in experiments, they are normally closely bound up with each other, so that fertilisation appears as a single, though complex, event (Fig. 3.1).		The moment of {{fertilisation}} is the conventional point of origin from which to date the existence of a new individual. We have seen in the last chapter that in fact many processes which are most important for the developing embryo occur before fertilisation, during the maturation of the egg. It cannot be denied, however, that fertilisation is, normally at least, the most crucial event within the continuous series of changes by which the new creature comes into being. It is not a simple occurrence, at which there is only one happening of importance; but its two important phases succeed one another quite quickly, and although they can be dissociated from one another in experiments, they are normally closely bound up with each other, so that fertilisation appears as a single, though complex, event (Fig. 3.1).


Line 25:		Line 25:

	All active substances produced by either eggs or sperm and acting on gametes of the opposite sort are sometimes collectively known as Gamones, the egg secretions being Gynogamones and the sperm secretions Androgamones. In this terminology, fertilizin becomes Gynogamone II and anti-fertilizin Androgamone II. The subject has recently been reviewed by Tyler (1948, 1949), Runnstrém (1949) and Rothschild (19514, b), and further details may be found in their papers.		All active substances produced by either eggs or sperm and acting on gametes of the opposite sort are sometimes collectively known as Gamones, the egg secretions being Gynogamones and the sperm secretions Androgamones. In this terminology, fertilizin becomes Gynogamone II and anti-fertilizin Androgamone II. The subject has recently been reviewed by Tyler (1948, 1949), Runnstrém (1949) and Rothschild (19514, b), and further details may be found in their papers.


	The actual process of fertilisation starts when the sperm first touches the egg surface. As mentioned above, the ensuing processes fall into two phases. These are:		The actual process of fertilisation starts when the sperm first touches the egg surface. As mentioned above, the ensuing processes fall into two phases. These are:
Line 30:		Line 31:
	(i) The activation of the egg. (ii) The union of the two haploid nuclei.		(i) The activation of the egg. (ii) The union of the two haploid nuclei.

	(1) Activation		==1. Activation==

	By ‘activation’ we mean the setting in train of a series of changes which bring the egg out of the quiescent state in which it awaited the arrival of a spermatozoan and start it off on the course of development. These changes take somewhat different forms in different groups, but there are certain common elements which are nearly always found. First in point of time, there may occur, in eggs surrounded by thick jelly, a reaction of the egg surface which assists the sperm in penetrating these outer coverings. For instance, in some echinoderm eggs, a conical projection pushes out from the egg surface and, as it were, catches hold of a sperm and draws it inwards through the jelly. Such happenings are, however, not found in all eggs.		By ‘activation’ we mean the setting in train of a series of changes which bring the egg out of the quiescent state in which it awaited the arrival of a spermatozoan and start it off on the course of development. These changes take somewhat different forms in different groups, but there are certain common elements which are nearly always found. First in point of time, there may occur, in eggs surrounded by thick jelly, a reaction of the egg surface which assists the sperm in penetrating these outer coverings. For instance, in some echinoderm eggs, a conical projection pushes out from the egg surface and, as it were, catches hold of a sperm and draws it inwards through the jelly. Such happenings are, however, not found in all eggs.


	Some kind of surface reaction of the egg is nearly always produced by the sperm. The most important and widespread form of the reaction is a change by which the first sperm which penetrates renders the egg surface impenetrable to later sperm. The exact nature of this change is still unknown; it may even differ in different groups. In many of the naked marine eggs, it is made visible by the formation at the surface of the egg of a new membrane, the ‘fertilisation membrane’, which lifts a little way off the egg immediately after activation (Runnstrém, 19524, b). This is very well seen in echinoderms; and in them it appears to be formed by the swelling and breaking up of a thin layer of colourless granules which can be found just below the surface of the ripe, unfertilised egg (Fig. 3.2). It seems, however, that in some species of echinoderms if not in all, the impact of the first spermatozoan causes a cortical change in the egg which spreads over the whole surface in a much shorter time than it takes for the fertilisation membrane to appear. Rothschild and Swann (1949) were able to reveal this change by the use of dark ground illumination, and they present some reasons for thinking that it is this almost immediate effect, rather than the relatively slow elevation of the fertilisation membrane, which constitutes the block to polyspermy.		Some kind of surface reaction of the egg is nearly always produced by the sperm. The most important and widespread form of the reaction is a change by which the first sperm which penetrates renders the egg surface impenetrable to later sperm. The exact nature of this change is still unknown; it may even differ in different groups. In many of the naked marine eggs, it is made visible by the formation at the surface of the egg of a new membrane, the ‘fertilisation membrane’, which lifts a little way off the egg immediately after activation (Runnstrém, 19524, b). This is very well seen in echinoderms; and in them it appears to be formed by the swelling and breaking up of a thin layer of colourless granules which can be found just below the surface of the ripe, unfertilised egg (Fig. 3.2). It seems, however, that in some species of echinoderms if not in all, the impact of the first spermatozoan causes a cortical change in the egg which spreads over the whole surface in a much shorter time than it takes for the fertilisation membrane to appear. Rothschild and Swann (1949) were able to reveal this change by the use of dark ground illumination, and they present some reasons for thinking that it is this almost immediate effect, rather than the relatively slow elevation of the fertilisation membrane, which constitutes the block to polyspermy.
Line 63:		Line 65:
	than that of unfertilised eggs, and it was thought that fertilisation brought about a great increase in respiration. However, more recent work has shown that the difference he described is due rather to a decrease in oxygen uptake by the stale unfertilised eggs than to an increase in the fertilised ones. Rothschild (19516), in a recent discussion of the metabolic changes produced by fertilisation, is very cautious about the extent of our knowledge on the subject. He suggests that the production of an acid of unknown nature is one of the most certain and striking phenomena, and lists a number of other changes, such as a reduction in glycogen content and a fall in respiratory quotient, without feeling justified in deciding which if any of these are of major importance.		than that of unfertilised eggs, and it was thought that fertilisation brought about a great increase in respiration. However, more recent work has shown that the difference he described is due rather to a decrease in oxygen uptake by the stale unfertilised eggs than to an increase in the fertilised ones. Rothschild (19516), in a recent discussion of the metabolic changes produced by fertilisation, is very cautious about the extent of our knowledge on the subject. He suggests that the production of an acid of unknown nature is one of the most certain and striking phenomena, and lists a number of other changes, such as a reduction in glycogen content and a fall in respiratory quotient, without feeling justified in deciding which if any of these are of major importance.

	(2) The union of the nuclei		==2. The union of the nuclei==

	The union of the two haploid nuclei of the egg and sperm is, from the long-term point of view, the most important phase of fertilisation. It is an essential part of the system of reproducing diploid adults by means of haploid gametes which has proved itself most efficient as an evolutionary mechanism and has therefore been perpetuated in the vast majority of animals and plants. The genetical and evolutionary aspects of the phenomenon fall outside our immediate field of interest. We must, however, give some account of the actual process by which the two nuclei come together.		The union of the two haploid nuclei of the egg and sperm is, from the long-term point of view, the most important phase of fertilisation. It is an essential part of the system of reproducing diploid adults by means of haploid gametes which has proved itself most efficient as an evolutionary mechanism and has therefore been perpetuated in the vast majority of animals and plants. The genetical and evolutionary aspects of the phenomenon fall outside our immediate field of interest. We must, however, give some account of the actual process by which the two nuclei come together.

Z8600021 at 02:25, 23 November 2019

2019-11-23T02:25:41Z

Show changes

Z8600021: Created page with "{{Header}} {{Ref-Waddington1956}} {{Waddington1956 TOC}} {{Waddington1956 TOC}} {{Footer}} Category:Historic EmbryologyCategory:1950'sCategory:Textbook"

2019-11-23T01:43:47Z

Created page with "{{Header}} {{Ref-Waddington1956}} {{Waddington1956 TOC}} {{Waddington1956 TOC}} {{Footer}} Category:Historic Embryology Category:1950's Category:Textbook"

New page

{{Header}}
{{Ref-Waddington1956}}
{{Waddington1956 TOC}}

{{Waddington1956 TOC}}
{{Footer}}
[[Category:Historic Embryology]][[Category:1950's]][[Category:Textbook]]

Waddington1956 3 - Revision history

Z8600021: /* Chapter III Fertilisation */

Z8600021: /* Chapter III Fertilisation */

Z8600021 at 02:25, 23 November 2019

Z8600021: Created page with "{{Header}} {{Ref-Waddington1956}} {{Waddington1956 TOC}} {{Waddington1956 TOC}} {{Footer}} Category:Historic EmbryologyCategory:1950'sCategory:Textbook"