Text-Book of Embryology 2-2 (1919) - Revision history

Z8600021 at 00:50, 16 Haziran 2018

2018-06-16T00:50:57Z

Z8600021 at 23:25, 15 Haziran 2018

2018-06-15T23:25:44Z

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	assumed a ventral position being overhung by the bulging tail rudiment, in D, E, 14‘, G the tail rudiment is seen to be extending actively past the position of the anus, the specially actively growing tissues being indicated by the darker shading.		assumed a ventral position being overhung by the bulging tail rudiment, in D, E, 14‘, G the tail rudiment is seen to be extending actively past the position of the anus, the specially actively growing tissues being indicated by the darker shading.

	In Fig. 80, G, a feature is well shown which occurs in the embryos of most Vertebrates—-—the postanal gut (pay). It was shown in Chap. I. how a connexion-—-the neurenteric canal--~~—~existed~~		In Fig. 80, G, a feature is well shown which occurs in the embryos of most Vertebrates—-—the postanal gut (pay). It was shown in Chap. I. how a connexion - the neurenteric canal - existed in some Vertebrates between the cavity of the enteron and that of the neural rudiment at their posterior ends. Here, in the postanal gut, we have such a connexion still persisting in a drawn-out form though, as in the present case, it may be a solid strand of yolky cells and not a hollow tube. The postanal gut is a purely transitory structure which at a relatively early period of development disintegrates completely.

	~~in some Vertebrates between the cavity of the enteron and that of.~~

	the ~~neural rudiment~~ at ~~their posterior ends~~. ~~Here~~, in the ~~postanal gut~~, we ~~have~~ such a ~~connexion still persisting~~ in ~~a drawn-out form though~~, ~~as in~~ the ~~present case~~, it ~~may be a solid strand of yolky cells and not a hollow tube~~. The ~~postanal gut~~ is a ~~purely transitory structure which at a relatively early period of development disintegrates completely~~.		In endeavouring to determine the morphological signiﬁcance of the postanal gut it is necessary to bear in mind that the Vertebrate in early stages develops from before backwards and that the growth in length by the addition of new segments takes place at its hinder end where there is a mass of actively growing embryonic tissue forming a kind of “growing point.” The tissue of this, although to the eye quite undifferentiated, contains the elements which form all the various tissues such as nerve cord, notochord, myotomes, alimentary canal, etc. As growth goes on these gradually become differentiated out, the differentiation always proceeding from before backwards. If we now look at such a young Vertebrate as that shown in Fig. 80, Or, we see the typical Vertebrate structure, including alimentary canal (pay) extending right back practically to the tip of the tail: it is only at the extreme tip that the various organs merge together into unditl‘erentiated embryonic tissue. The only striking peculiarity is that the communication of the alimentary canal with the exterior, the anus, is not in the midst of the growing tissue of the tip, as it would be, for example, in a young Chaetopod worm, but well forwards on the ventral side.

	In endeavouring to determine the morphological signiﬁcance of the postanal gut it is necessary to bear in mind that the Vertebrate in early stages develops from before backwards and that the growth in length by the addition of new segments takes place at its hinder end where there is a mass of actively growing embryonic tissue forming a kind of “growing point.” The tissue of this, although to the eye quite undifferentiated, contains the elements which form all the various tissues such as nerve cord, notochord, myotomes, alimentary canal, etc. As growth goes on these gradually become differentiated out, the differentiation always proceeding from before backwards. If we now look at such a young Vertebrate as that shown in Fig. 80, Or, we see the typical Vertebrate structure, including alimentary canal (pay) extending right back practically to the tip of the tail: it is only at the extreme tip that the various organs merge together into unditl‘erentiated embryonic tissue. The only striking peculiarity is that the communication of the alimentary canal with the exterior, the anus, is not in the midst of the growing tissue of the tip, as it would be, for example, in a young Chaetopod worm, but well forwards on the ventral side.

	This peculiarity, in the writer’s opinion, ﬁnds its explanation in the development from before backwards already alluded to. The appearance of the anus at a point relatively far forwards means that it and the organs related to it such as the excretory d11cts complete their development at an earlier period of time. As it is of functional importance that the organs in question should do so, in contradistinction to the purely motor arrangements farther back, we see a physiological reason why evolution should have brought about a development of the anal opening in its anterior position from the beginning, and the elimination of those stages in which it was situated farther back.		This peculiarity, in the writer’s opinion, ﬁnds its explanation in the development from before backwards already alluded to. The appearance of the anus at a point relatively far forwards means that it and the organs related to it such as the excretory d11cts complete their development at an earlier period of time. As it is of functional importance that the organs in question should do so, in contradistinction to the purely motor arrangements farther back, we see a physiological reason why evolution should have brought about a development of the anal opening in its anterior position from the beginning, and the elimination of those stages in which it was situated farther back.

Z8600021 at 22:52, 15 Haziran 2018

2018-06-15T22:52:41Z

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	~~{{Header}}~~
	{{Kerr1919 header}}		{{Kerr1919 header}}

	=Chapter II The Skin And Its Derivatives=		=Chapter II The Skin And Its Derivatives=
	The skin of the vertebrate consists of the epidermis—the persistent and less or more modiﬁed ectoderm—resting upon the superficial layer of mesenchy1ne—the dermis—which in the higher forms becomes strengthened by the formation of numerous tough interlacing ﬁbres.		The skin of the vertebrate consists of the epidermis—the persistent and less or more modiﬁed ectoderm—resting upon the superficial layer of mesenchy1ne—the dermis—which in the higher forms becomes strengthened by the formation of numerous tough interlacing ﬁbres.

Z8600021 at 22:52, 15 Haziran 2018

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			{{Header}}
			{{Kerr1919 header}}
	=Chapter II The Skin And Its Derivatives=		=Chapter II The Skin And Its Derivatives=
	The skin of the vertebrate consists of the epidermis—the persistent and less or more modiﬁed ectoderm—resting upon the superficial layer of mesenchy1ne—the dermis—which in the higher forms becomes strengthened by the formation of numerous tough interlacing ﬁbres.		The skin of the vertebrate consists of the epidermis—the persistent and less or more modiﬁed ectoderm—resting upon the superficial layer of mesenchy1ne—the dermis—which in the higher forms becomes strengthened by the formation of numerous tough interlacing ﬁbres.

Z8600021 at 22:51, 15 Haziran 2018

2018-06-15T22:51:35Z

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			=Chapter II The Skin And Its Derivatives=

	The skin of the vertebrate consists of the epidermis—the persistent and less or more modiﬁed ectoderm—resting upon the superficial layer of mesenchy1ne—the dermis—which in the higher forms becomes strengthened by the formation of numerous tough interlacing ﬁbres.		The skin of the vertebrate consists of the epidermis—the persistent and less or more modiﬁed ectoderm—resting upon the superficial layer of mesenchy1ne—the dermis—which in the higher forms becomes strengthened by the formation of numerous tough interlacing ﬁbres.

Z8600021 at 22:50, 15 Haziran 2018

2018-06-15T22:50:42Z

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	~~THE~~ skin of the vertebrate consists of the epidermis—the persistent and less or more modiﬁed ectoderm—resting upon the superficial layer of mesenchy1ne—the dermis—which in the higher forms becomes strengthened by the formation of numerous tough interlacing ﬁbres.

			The skin of the vertebrate consists of the epidermis—the persistent and less or more modiﬁed ectoderm—resting upon the superficial layer of mesenchy1ne—the dermis—which in the higher forms becomes strengthened by the formation of numerous tough interlacing ﬁbres.

	In studying the development of the skin in the various types of vertebrate we find that the ectoderm undergoes characteristic modiﬁcations to ﬁt it for the carrying out of special functions. In the ﬁshes it becomes converted into a highly glandular mechanism concerned with the production of slippery mucus for the diminution of what the naval architect calls “skin-friction,” in other words the friction between the surface of the body and the water in contact with it. Local or general specializations of this glandular apparatus lead to the development of cement organs by the secretion of which the young animal is able to attach itself to solid supports, to the production of digestive ferments by which the eggshell is softened or, in the case of the portion of ectoderm which lines the buccal cavity, the digestion of the food initiated, or to the production of poisonous defensive or offensive secretions. In the case of the terrestrial amphibians the glandular apparatus serves to keep the skin moist, while in the Birds it develops arrangements for oiling the feathers.		In studying the development of the skin in the various types of vertebrate we find that the ectoderm undergoes characteristic modiﬁcations to ﬁt it for the carrying out of special functions. In the ﬁshes it becomes converted into a highly glandular mechanism concerned with the production of slippery mucus for the diminution of what the naval architect calls “skin-friction,” in other words the friction between the surface of the body and the water in contact with it. Local or general specializations of this glandular apparatus lead to the development of cement organs by the secretion of which the young animal is able to attach itself to solid supports, to the production of digestive ferments by which the eggshell is softened or, in the case of the portion of ectoderm which lines the buccal cavity, the digestion of the food initiated, or to the production of poisonous defensive or offensive secretions. In the case of the terrestrial amphibians the glandular apparatus serves to keep the skin moist, while in the Birds it develops arrangements for oiling the feathers.
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	==The Epidermis==		==The Epidermis==

	The ~~cctoderm~~ covering the surface of the embryo becomes converted, normally, into the epidermis of the fully developed individual. Very usually the embryonic ectoderm consists of two layers of cells, the lower layer composed of actively living cells, the superﬁcial of ﬂattened plate-like protective cells. This outer layer has been termed by Krause the periderm: its superﬁcial protoplasm is commonly hardened to form a cuticle in the strict sense of the term. Normally it plays no active part in development a11d is shed at an early period.		The ectoderm covering the surface of the embryo becomes converted, normally, into the epidermis of the fully developed individual. Very usually the embryonic ectoderm consists of two layers of cells, the lower layer composed of actively living cells, the superﬁcial of ﬂattened plate-like protective cells. This outer layer has been termed by Krause the periderm: its superﬁcial protoplasm is commonly hardened to form a cuticle in the strict sense of the term. Normally it plays no active part in development a11d is shed at an early period.

	The deep layer of the ectoderm on the other hand is active. Its cells multiply so that it becomes several layers thick: the outer layers become corniﬁed to form the horny stratum of the epidermis while the deeper cells, composed of active living protoplasm, form the stratum of Malpighi.		The deep layer of the ectoderm on the other hand is active. Its cells multiply so that it becomes several layers thick: the outer layers become corniﬁed to form the horny stratum of the epidermis while the deeper cells, composed of active living protoplasm, form the stratum of Malpighi.

Z8600021 at 22:22, 15 Haziran 2018

2018-06-15T22:22:31Z

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CHAPTER II THE SKIN AND ITS DERIVATIVES

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