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HANDBOOK OF PHYSIOLOGY. 



to the parietes of the trunk, while the latter adheres more or less closely 

 to the hypoblast, and gives rise to the serous and muscular walls of the 

 alimentary canal and several other parts (Fig. 450). 



The united parietal layer of the mesoblast with the epiblast is termed 

 Somatopleure, the united visceral layer and hypoblast, Splanchno- 

 pleure. The space between them is the pleuro-peritoneal cavity, 

 which becomes subdivided by subsequent partitions into pericardium, 

 pleura, and peritoneum. 



FIG. 450. Transverse section through dorsal region of embryo chick (45 hrs.). One-half of 

 the section is represented: if completed it would extend as far to the left as to the right of the line 

 of the medullary canal (A/c). A, epiblast; C, hypoblast, consisting of a single layer of flattened 

 cells; Me, medullary canal; Pv, proto vertebra; Wd, Wolfflan duct: So, somatapleure; Sp, splanch- 

 nopleure; pp, pleuro-peritoneal cavity; ch, notochord; ao, dorsal aorta, containing blood-cells; v, 

 blood-vessels of the yolk-sac (Foster and Balfour). 



The splitting of the mesoblast extends almost to the medullary canal, 

 but a portion on either side ( p. v. Fig. 450) remains undivided, the 

 vertebral plate. The divided portion is known as the lateral plate. 

 The longitudinal thickening of the vertebral plate is seen after awhile 

 to be divided, at right angles to the medullary canal by bright trans- 



FIG. 451. Diagrammatic longitudinal section through the axis of an embryo. The head-fold 

 has commenced, but the tail-fold has not yet appeared; FSo, fold of the somatopleure; FSp, fold 

 of the splanchnopleure ; the line of reference, FSo, lies outside the embryo in the "moat," which 

 marks off the overhanging head from the amnion; D, inside the embryo, is that part which is to 

 become the fore-gut; FSo and Fsp, are both parts of the head-fold, and travel to the left of the 

 figure as development proceeds; op, space between somatopleure and splanchnopleure, pleuro-peri- 

 toneal cavity; Am, commencing head-fold of amnion; NC, neural canal; Ch, notochord; Ht, heart; 

 A, B, C, epiblast, mesoblast, hypoblast (Foster and Balfour). 



verse lines into a number of square segments. These segments, which 

 are the surface appearance of cubes of mesoblast, are the mesoblastic 

 somites or protovertebrae. The first three or four of these proto- 

 vertebrae make their appearance in the cervical region, while one or two 



