DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK. 



masses of them are, however, far smaller and far fewer than 

 in the posterior part of the embryo. It is at the tail end of 

 the pellucid area that the chief formation of blood-corpuscles 

 takes place. 



The part of the pellucid area intermediate in position be- 

 tween the anterior and posterior ends of the embryo is likewise 

 intermediate as regards the number of corpuscles formed and 

 the size of the spaces between the processes ; the spaces being 

 here larger than at the posterior extremity, but smaller than 

 the spaces in front. Close to the sides of the embryo the spaces 

 are, however, smaller than in any other part of the pellucid 

 area. It is, however, in this part that the first formation of 

 blood-corpuscles takes place, and that the first complete capil- 

 laries are formed. 



We have then somewhat round protoplasmic masses filled 

 with blood-corpuscles and connected by means of processes, a 

 few of which may begin to contain blood-corpuscles, but the 

 majority of which only contain ordinary nuclei. The next 

 changes to be noticed take place in the nuclei which were not 

 converted into blood-corpuscles, but which were to be seen in 

 the protoplasm surrounding the corpuscles. They become more 

 numerous and smaller, and, uniting with the protoplasm in 

 which they were imbedded, become converted into flat cells 

 (spindle-shaped in section), and in a short time form an entire 

 investment for the masses of blood-corpuscles. The same 

 change also occurs in the protoplasmic processes which con- 

 nect the masses of corpuscles. In the case of those processes 

 which contain no corpuscles the greater part of their protoplasm 

 seems to be converted into the protoplasm of the spindle-shaped 

 cells. The nuclei arrange themselves so as completely to sur- 

 round the exterior of the protoplasmic processes. In this way 

 each process becomes converted into a hollow tube, completely 

 closed in by cells formed from the investment of the original 

 nuclei by the protoplasm which previously formed the solid 

 processes. The remainder of the protoplasm probably becomes 

 fluid, and afterwards forms the plasma in which the corpuscles 

 float. While these changes are taking place the formation of 

 the blood-corpuscles does not stand still, and by the time a 

 system of vessels, enclosed by cellular walls, is formed out of 



42 



