886 REPKODUCTION, DEVELOPMENT, GKOWTH AND DEATH [CH. LIX. 



primitive skeletal axis. It soon separates from the remainder of the 

 hypoblast, and forms a round cord, which lies at first immediately 

 beneath the neural groove, and afterwards beneath the neural tube, 

 extending from the anterior end of the primitive gut, which lies 

 beneath that region of the neural tube which afterwards becomes 

 the mid-brain, to the caudal end of the embryo (figs. 571, 572). The 

 notochord is subsequently obliterated by the formation of the verte- 

 brae from the protovertebral somites. 



14 7 



FIG. 571. Diagram of a transverse section 

 through a mammalian embryo at the 

 period when the folding off of the fetus 

 has commenced. 1, Neural tube ; 2, 

 protovertebral somite ; 3, epiblast ; 4, 

 somatic mesoblast ; 5, splanchnic meso- 

 blast ; 6, hypoblast ; 7, notochord ; 8, 

 primitive alimentary canal ; 9, coelom ; 

 10, vitello-intestinal duct ; 11, yolk-sac ; 

 12, lateral fold of amnion. 



FIG. 572. Diagram of a longitudinal section of a 

 mammalian embryo at the period when the folding 

 off of the foetus has commenced. 1, Neural tube ; 

 2, epiblast ; 3, notochord ; 4, stomadoeai space ; 5, 

 head fold of amnion ; 6, tail fold of amnion ; 7, 

 hypoblast ; 8, somatic mesoblast ; 9, splanchnic 

 mesoblast ; 10, yolk-sac ; 11, coelom ; 12, allantois ; 

 13, hind-gut ; 14, mid-gut ; 15, fore-gut; 16, peri- 

 cardium. 



The foetus contains parts of all the layers of the blastoderm, and 

 from each of these certain organs are formed as indicated in the 

 following list: 



1. From Epiblast. a. The epidermis and its appendages. 

 &. The nervous system, both central and peripheral. 



c. The epithelial structures of the sense-organs. 



d. The epithelium of the mouth, and the enamel of the teeth. 



e. The epithelium of the nasal passages. 



/. The epithelium of the glands opening on the skin and into the 

 vestibule of the mouth, and nasal passages. 

 g. The muscular fibres of the sweat-glands. 



2. Prom Mesoblast. a. The skeleton and all the connective 

 tissues of the body. 



b. All the muscles of the body (except those of the sweat-glands). 



c. The vascular system, including the lymphatics, serous mem- 

 branes, and spleen. 



