DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK. 49 



processes. Whether they are carried out into the processes 

 by the movement of the surrounding protoplasm, or whether 

 they move through the protoplasm, I have been unable to 

 determine ; the former view, however, seems to be the most 

 probable. 



It is possible that some nuclei arise spontaneously in the 

 protoplasm, but I am much more inclined to think that they 

 are all formed by the division of pre-existing nuclei a view 

 favoured by the number of nuclei which are seen to possess two 

 nucleoli. Coincidently with the formation of the new nuclei 

 the protoplasm of the processes, as well as that surrounding the 

 nuclei at the starting-points of the processes, begins to increase 

 in quantity. 



At these points the nuclei also increase more rapidly than 

 elsewhere, but at first the resulting nuclei seem to be all of the 

 same kind. 



In the anterior part of the pellucid area (fig. 4) the increase 

 in the number of nuclei and in the amount of protoplasm at the 

 starting-points of the protoplasm is not very great, but in the 

 posterior part the increase in the amount of the protoplasm at 

 these points is very marked, and coincidently the increase in 

 number of the nuclei is also great. This is shewn in figs. 2 

 and 3. These are both taken from the tail end of an embryo 

 of about thirty-three hours, with seven or eight proto-vertebrae. 

 Fig. 3 shews the processes beginning to increase in thickness, 

 and also the protoplasm at the starting-points increasing in 

 quantity ; at the same time the nuclei at these points are be- 

 ginning to become more numerous. Fig. 3 is taken from a 

 slightly higher level, i. e. slightly nearer the epiblast. In it 

 the protoplasm is seen to have increased still more in quantity, 

 and to be filled with nuclei. These nuclei have begun to be 

 slightly coloured, and one of them is seen to possess two 

 nucleoli. 



Very soon after this a change in the nuclei begins to be 

 observed, more especially in the hinder part of the embryo. 

 While before this time they were generally elongated, some of 

 them now become more nearly circular. In addition to this, 

 they begin to have a yellowish tinge, and the nuclei, when 

 treated with gold (for in the fresh condition it is not easy to 

 B. 4 



