108 



DE VELOPMENT. 



muscles and true skin. On either side of the protovertebrse the mesoblast splits 

 into two layers, the upper, or that covered by epiblast, is called somatopleure, and 

 the lower, lined by hypoblast, the splanchnopleure (Fig. 82, B, 5-5'). From the 



FIG. 81. Embryonic area of the ovum of a rabbit at the 

 seventh day. ay. Embryonic area, o, o. Region of the 

 blastodermic vesicle immediately surrounding the embry 

 onic area. pr. Primitive streak, rf. Medullary groove 

 (From Kolliker.) 



former the skeleton muscles and true skin of the 

 external parts of the body are derived, from the I at 



FIG. 82. Transverse sections 

 through the embryo-chick before 

 and some time after the closure of 

 the medullary canal, to show the 

 upward and downward inflections 

 of the blastoderm. (After Remak.) 

 A. At the end of the first day. 1. 

 Notochord. 2. Primitive groove in 

 the medullary canal. 3. Edge of the 

 dorsal lamina. 4. Epiblast. 5. Meso- 

 blast divided in its inner part. 6. 

 Hypoblast. 7. Section bfprotoverte- 

 bral plate. B. On the third day in 

 the lumbar region. 1. Notochord in 

 its sheath. 2. Medullary canal now 

 closed in. 3. Section of the medul- 

 lary substance of the spinal cord. 

 4. Epiblast. 5. Somatopleure of 

 the mesoblast. 5'. Splanchnopleure 

 (one figure is placed in the pleuro- 

 peritoneal cavity). 6. Layers of hy- 

 poblast in the intestines spreading 

 also over the yolk. 4X5. Part of the 

 fold of the amnion formed by epi- 

 blast and somatopleure. 



FIG. 83. Diagrammatic section through the ovum of a mam- 

 mal in the long axis of the embryo, e. The cranio-vertebral axis 

 /, i. The cephalic and caudal portions of the primitive alimentary 

 canal, a. The amnion. a'. The point of reflection into the false 

 amnion. v. Yolk-sac, communicating with the middle part of 

 the intestine by v i, the vitello-intestinal duct. u. The allantois. 

 The ovum is surrounded externally by the villous chorion. 



aptly compared to a canoe turned over (Fig. 83). 



ter, the muscular and other 

 mesoblastic portions of the vis- 

 cera. The space between them 

 is the common pleuro-peritoneal 

 cavity. Whilst the parietes of 

 the body are still unclosed, this 

 common pleuro-peritoneal cav- 

 ity is continuous with the space 

 between the amnion and cho- 

 rion, as seen in Fig. 79, F. The 

 embryo, which at first seems to 

 be a mere streak, extends longi- 

 tudinally and laterally. As it 

 grows forward the cephalic end 

 becomes remarkably curved on 

 itself (cephalic flexure), and 

 a smaller but distinct flexure 

 takes place at its hinder end 

 (caudal flexure). At the same 

 time the sides of the embryo 

 grow and curve toward each 

 other ; so that the embryo is 

 In consequence of this incurv- 



