INTRODUCTION 



11 



Ulaierally symmetrical. The neural tube, or cerebro-spinal cavity, 

 enclosed by the skull and vertebral arches, contains the central ner- 

 TTOUS system {hrain and spinal cord) ; the visceral tube (ccelome, 

 p. 8, Fig. 7) encloses the viscera (alimentary canal, urinogenital 

 -organs, &c.),and its muscular walls may be strengthened by a series 



l'f(CLf) 



Fig. 9. — Diagrammatic Section through the Human Gravid Uteris. 



U, uterus ; Th, Tb, Fallopian tubes ; UH, uterine cavity ; Dv, deeitlua vera, which 

 at Pu passes into the uterine portion of the placenta ; Dr, decidua reflexa ; 

 Pf, fcutal portion of the placenta (chorion frondosum, Chf) ; Chi, chorion 

 liKve lAtA, the cavity of the amnion filled with iluid : in the interior of the 

 amnion is seen the embryo suspended by the twisted umbilical cord ; H, 

 neart ; A, aorta ; cs, precaval, ci, postcaval, and p, portal vein ; Al, allantoic 

 (umbilical) arteries ; t, the liver, perforated by the umbilical vein ; D, the 

 remains of the yolk-sac (umbilical vesicle). 



of ribs, articulating dorsally with the vertebral column. Certain 

 of the ribs may reach the mid-ventral line and come into connec- 

 tion with a breast-bone or sternum, and thus form complete rings 

 •or hoops around the visceral tube. 



The anterior ends of the central nervous system (brain) and ali- 

 mentary tract enter into close relations with the outer world, the 



