INTRODUCTION. 9 



side of the epiblast, and into a visceral layer (SpP\ which 

 becomes attached to the hypoblast. The former, together with 

 the epiblast to which it is united, constitutes the somatopleure, 

 and the latter, together with the hypoblast, the splanchnopleure. 

 The cavity separating these is the body-cavity, or coelome. 

 The upper part of the mesoblast, or that which lies on either side 

 of the medullary cord and notochord (see p. 10), becomes separated 

 from the lower, and segmented to form a series of mesoblastic 



fad 



KW 



FIG. 8. DIAGRAMMATIC TRANSVERSE SECTION- THROUGH THE BODY OF AN ADULT 



VERTEBRATE. 



Med, spinal cord ; Nil, neural tube ; KW, body-wall ; Co, derma ; Ep, epidermis ; 

 VII, visceral tube ; Ao, aorta ; Ms, mesentery ; Per, parietal layer of the peri- 

 toneum ; Per 1 , visceral layer of the peritoneum ; Msc, musculature of intes- 

 tine ; Subm, connective-tissue coat of the intestine ; Ep, epithelium of intestine ; 

 DH, lumen of intestine ; W, vertebral column. 



somites, or protovertebrae (Fig. 7, B, UW, H}, which lose 

 their cavities, and eventually give rise to the vertebral column 

 and longitudinal lateral muscles. 



The Vertebrate body is formed on a bilaterally symmetrical 

 plan, and it may be described as consisting in the adult of two 

 tubes, a dorsal and a ventral (Fig. 8, XR, VR). The former, or 

 ceiebro-spinal cavity, encloses the central nervous system (brain 

 and spinal cord), and may therefore be spoken of as the neural 



1 The ccelome may arise as a segmentaUy arranged series of outgrowths from the 

 archenteron, as, e.g., in Amphioxus (enterocceles), or it may be formed secondarily 

 by a splitting (delamination) of the mesoblastic tissue (schizocoele). The first o'f 

 these must be considered as the most primitive. 



