Waddington1956 9 - Revision history

Z8600021: /* (a) Amphibia */

2019-11-23T06:59:01Z

(a) Amphibia

← Older revision		Revision as of 17:59, 23 November 2019
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	(b) Birds		===(b) Birds===

	Although the gastrulation movements in birds are more complex than in Amphibia, they are perhaps easier to visualise and to describe, since they occur not in a sphere, but in a flat circular disc. The main complication which is introduced is a double streaming movement along the midline, forwards at an early stage, and backwards later on. There is moreover another important difference from the Amphibia in connection with the type of growth which is proceeding. In the latter group the total mass of the egg is more or less constant during gastrulation; ‘growth’ consists in the divison of cells into smaller units, and the conversion into living substance of yolk, already contained in the cells. In the birds, on the other hand, the greater part of the yolk lies outside the tissues, and during gastrulation this is being digested and assimilated, so that growth involves an actual increase in the cellular mass.		Although the {{gastrulation}} movements in birds are more complex than in Amphibia, they are perhaps easier to visualise and to describe, since they occur not in a sphere, but in a flat circular disc. The main complication which is introduced is a double streaming movement along the midline, forwards at an early stage, and backwards later on. There is moreover another important difference from the Amphibia in connection with the type of growth which is proceeding. In the latter group the total mass of the egg is more or less constant during gastrulation; ‘growth’ consists in the divison of cells into smaller units, and the conversion into living substance of yolk, already contained in the cells. In the birds, on the other hand, the greater part of the yolk lies outside the tissues, and during gastrulation this is being digested and assimilated, so that growth involves an actual increase in the cellular mass.

Z8600021: /* Chapter IX The Vertebrates: The Amphibia and Birds */

2019-11-23T06:57:55Z

Chapter IX The Vertebrates: The Amphibia and Birds

← Older revision		Revision as of 17:57, 23 November 2019
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	1. From the unfertilised egg to the formation of the blastula (a) The Amphibia The fully grown frog’s or newt’s egg is a fairly large spherical cell, some 2 or 3 mm. in diameter. It is usually seen after being extruded into the water; normally the eggs are fertilised either as they leave the female’s body (in frogs) or by sperm which the female has taken up into her cloaca (in newts). The egg within the ovary and oviduct is surrounded by a viscid layer of jelly, which swells on contact with water into a thick protective covering; in newts each egg is enclosed within a separate capsule, but in frogs a whole clutch of eggs coheres into a single mass within which the individual eggs are scattered.		==1. From the unfertilised egg to the formation of the blastula==

			===(a) The Amphibia===

			The fully grown frog’s or newt’s egg is a fairly large spherical cell, some 2 or 3 mm. in diameter. It is usually seen after being extruded into the water; normally the eggs are fertilised either as they leave the female’s body (in frogs) or by sperm which the female has taken up into her cloaca (in newts). The egg within the ovary and oviduct is surrounded by a viscid layer of jelly, which swells on contact with water into a thick protective covering; in newts each egg is enclosed within a separate capsule, but in frogs a whole clutch of eggs coheres into a single mass within which the individual eggs are scattered.

Z8600021 at 06:57, 23 November 2019

2019-11-23T06:57:06Z

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2019-11-23T01:43:20Z

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