. Cunningham's Text-book of anatomy. Anatomy. THE PRIMITIVE ALIMENTARY CANAL. 41 The nails are later developments. They appear at the third month and reach the ends of the digits at the sixth month. Each limb bud is essentially an extension of a definite number of segments of the body. It consists, at first, of a core of mesoderm covered by ectoderm. As it grows the anterior branches of the spinal nerves of the corresponding segments are prolonged into it, together with a number of blood- vessels. The nerves remain as the nerves of the fully developed limb, but the blood-vessels are reduced in
. Cunningham's Text-book of anatomy. Anatomy. THE PRIMITIVE ALIMENTARY CANAL. 41 The nails are later developments. They appear at the third month and reach the ends of the digits at the sixth month. Each limb bud is essentially an extension of a definite number of segments of the body. It consists, at first, of a core of mesoderm covered by ectoderm. As it grows the anterior branches of the spinal nerves of the corresponding segments are prolonged into it, together with a number of blood- vessels. The nerves remain as the nerves of the fully developed limb, but the blood-vessels are reduced in number and are modified until a smaller number of permanent main trunks is established. The greater part, if not the whole, of the mesodermal core of the primitive limb-rudiment seems to be produced by the somatic mesoderm of the lateral plate. As the development proceeds it is differentiated into the cartilagin- ous, muscular, and other connective tissue elements which are the rudi- ments of the skeletal framework and the muscles and fasciae of the adult limb. It is not yet decided whether or not the muscle elements of the mesodermal core are derived from the lateral plate mesoderm or from muscle cells which have migrated into the limb, from the muscle plates of the segments from which the limb is formed and from which muscles of the body wall are developed; and although it is generally believed that the bone which replaces the cartilaginous skeletal rudiments is produced by mesodermal cells, it has been asserted that the bone-producing cells originate in the ectoderm and migrate from the surface into the Fig. 54.—Lateral View of a Human* Embryo—9-5 mm. Long. (Keibel and Elze, X&rmaUafeln.) Note that the limb rudiments no longer project at right angles from the side of the body but that they are bent veutrally. THE EARLIER MODIFICATIONS OF THE PRIMITIVE ENTODERMAL ALIMENTARY CANAL AND THE FORMATION OF THE STOMA- TODiEUM AND PROCTODEUM. The greater part of t
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectanatomy, bookyear1914
