THE PEIMITIYE ALIMENTAEY 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 interior. 



FIG. 54. LATERAL VIEW OP A HUMAN EMBRYO 9 '5 mm. 

 LONG. (Keibel and Elze, Normaltafeln.) 



Note that the limb rudiments no longer project at right angles 

 from the side of the body but that they are bent ventrally. 



THE EAELIEE MODIFICATIONS OF THE PEIMITIVE ENTODEEMAL 

 ALIMENTAEY CANAL AND THE FOEMATION OF THE STOMA- 

 TODJEUM AND PEOCTOD^EUM. 



The greater part of the permanent alimentary canal is derived from the ento- 

 dermal sac and is therefore lined by entoderm cells. This part is enclosed in the 

 embryo as the latter is folded off from the remainder of the zygote (Fig. 50), but 

 the cephalic and caudal portions of the alimentary canal are formed by the 

 enclosure of part of the external space and are, therefore, lined by ectoderm. 



The cephalic part is a portion of a space called the stomatodseum which lies, 

 at first, between the ventrally bent extremity of the head and the bulging 

 pericardial region (Fig. 50). At a later period it is enclosed laterally by the rudi- 

 ments of the maxillae or upper jaws, and caudally by the mandibular rudiments. 



When it first appears the stomatodseum is separated from the cephalic end of 

 the entodermal portion of the primitive canal by the bucco-pharyngeal membrane, 

 but when that septum disappears, during the third week, the stomatodseum 

 communicates with the fore-gut. Subsequently, it is separated into nasal and 

 oral portions, and the oral portion forms that part of the mouth in which the gums 

 and teeth are developed. 



The caudal part of the permanent canal is formed by the elevation of a surface 



