THE DEVELOPMENT OF THE FROG 271 



splanchriopleur. They now grow down and unite below, and 

 at the same time their lower borders are split so that two 

 cavities are formed separated in the middle line by a partition. 

 These cavities are a part of the general ccelom and are con- 

 tinuous with it posteriorly. A space is left between the 

 splanchnopleur and the ventral wall of the throat in which a 

 few scattered irregular cells, derived from the mesoblast, make 

 their appearance.* After a time these cells unite to form a 



Fig. 67 



A, £, and C. Transverse sections through the ventral wall of the throat of embryos 

 of different ages, showing successive stages in the development of the heart. 

 e^, epiblast ; /t^y, hypoblast ; wes, mesoblast ; e^/t, endothelial lining of heart ; 

 Af, heart ; ^c, pericardial cavity ; s, sucker ; sc, somatic layer of mesoblast ; 

 j^/, splanchnic layer of mesoblast, eventually forming the wall of the heart. 



tube, and become the endothelial lining of the heart. Mean- 

 while the inner or splanchnopleuric layer becomes thickened, 

 and its sides are folded in towards the centre so as to enclose 

 the scattered cells. As the endothelial tube is formed, the 

 folds meet and unite above it, as is shown in fig. 67, so that 



* I am unable to find any support for the view, maintained by some 

 authors, that these cells are formed from the hypoblast. 



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