Fig. 44. Outline of a Human Embryo 10.4 mm long and in the 6th week of development
(After Broman.) (Mag. x 5.)
External Changes in the 6th week
As may be seen by comparing Figs. 42 and 44, the 6th week constitutes a period of rapid transformation.
Not only does the length of the embryo increase from 5 mm to 11 mm but there are very definite changes in the form of its external parts. At the end of the 5th week the gill-arch system of the primitive pharynx is at its height, four arches being distinguishable ; in the 6th week the 3rd and ith arches sink into a pit in the neck — the cervical sinus — (Fig. 44), while the 2nd or hyoid arch grows backwards over the pit and thus hides the hinder arches. Here we are witnessing the closing in or operculation of the branchial arches — as it takes place in gill-bearing vertebrates. Even at the close of the 6th week the face is represented merely by a forehead filled out by the relatively small f orebrain vesicle ; behind and under the forehead are seen the nasal, maxillary and mandibular processes which will give rise to the face proper. All of these elements have made headway during the 6th week (Figs. 42, 43, 44). The head region even in the 6th week is still tubular in form ; the mid and hind brains form the greater part of the central nervous system, for the cerebral vesicles have as yet only begun to grow out from the fore brain. The limb buds, which in the 5th week were still undemarcated into segments, now show their three primitive parts — upper arm and thigh, forearm and leg, and a platelike hand and foot. In point of differentiation the forelimb is always in advance of the hind. At the close of the 6th week the first appearance of jvebbed digits can be detected, and at the same time, when the length of the embryo is about 11 mm. the tail reaches its maximum development (Fig. 44) ; in the 7th week retrogression has already set in. The umbilical cord becomes lengthened and more clearly differentiated in the 6th week ; between the attachment of the cord to the ventral wall of the embryo and the gill-formation of the primitive pharynx is seen the bulging eminence of the heart (Fig. 42) ; below the heart eminence, as may be seen in Fig. 44, there appears in the 6th week a second eminence, that caused by the developing liver.
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Keith A. Human Embryology and Morphology. (1921) New York, Longmans, Green & Co. London: Edward Arnold.
Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures
Cite this page: Hill, M.A. (2019, July 19) Embryology Keith1921 fig044.jpg. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/File:Keith1921_fig044.jpg
- © Dr Mark Hill 2019, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
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