DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK. 55 



In the vessels themselves the coloured corpuscles undergo 

 increase by division, as has already been shewn by Remak. 

 Corpuscles in the various stages of division may easily be found. 

 They do not appear to show very active amoeboid movements in 

 the vessels, though their movements are sometimes very active 

 when removed from the body. 



To recapitulate some of the cells of the mesoblast of the 

 splanchnopleure send out processes, these processes unite with 

 the processes from other cells, and in this way a network is 

 formed. The nuclei of the original cells divide, and at the 

 points from which the processes start their division is especially 

 rapid. Some of them acquire especially at these points a red 

 colour, and so become converted into blood-corpuscles ; the 

 others, together with part of the protoplasm in which they are 

 imbedded, become converted into an endothelium both for the 

 processes and the masses of corpuscles ; the remaining proto- 

 plasm becomes fluid, and thus the original network of the cells 

 becomes converted into a network of hollow vessels, filled with 

 fluid, in which corpuscles float. 



In reference to the development of the heart, my observa- 

 tions are not quite complete. It is, however, easy to prove 

 from sections (vide figs. 10 and II, PI. 2) that the cavity of the 

 heart is produced by a splitting or absorption of central cells 

 of the thickened mesoblast of the splanchnopleure, while its 

 muscular walls are formed from the remaining cells of this 

 thickened portion. It is produced in the following way : 

 When the hypoblast is folded in to form the alimentary canal 

 the mesoblast of the splanchnopleure follows it closely, and 

 where the splanchnopleure turns round to assume its normal 

 direction (fig. 11) its mesoblast becomes thickened. This thick- 

 ened mass of mesoblast is, as can easily be seen from figs. 10 

 and 11, PI. 2, entirely distinct from the mesoblast which forms 

 the outside walls of the alimentary canal. At the point where 

 this thickening occurs an absorption takes place to form the 

 cavity of the heart. The method in which the cavity is formed 

 can easily be seen from figs. 10 and n. It is in fig. 11 shewn 

 as it takes place in the mesoblast on each side, the folds 

 of the splanchnopleure not having united in the middle line ; 

 and hence a pair of cavities are formed, one on each side. It 



