50 DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK.. 



see them distinctly), have a more jagged and irregular appear- 

 ance than the nucleoli of the other nuclei. 



This change takes place especially at the starting-points of 

 the processes, so that the appearance presented (fig. 5) is that 

 of spherical masses of yellowish nuclei connected with other 

 similar spherical masses by protoplasmic processes, in which 

 nuclei of the original type are seen imbedded. These masses 

 are surrounded by a thin layer of protoplasm, at the edge of 

 which a normal nucleus may here and there be detected, as at 

 fig. 5 a and a', the latter possessing two nucleoli. Some of 

 these processes are still very delicate, and it is exceedingly 

 probable that they undergo further changes of position before 

 the final capillary system is formed. 



These differentiated nuclei are the first stage in the forma- 

 tion of the blood-corpuscles. From their mode of formation 

 it is clear that the blood-corpuscles of the Sauropsida are to be 

 looked upon as nuclei containing nucleoli, rather than as cells 

 containing nuclei ; indeed, they seem to be merely ordinary 

 nuclei with red colouring matter. 



This would make them truly instead of only functionally 

 homologous with the red corpuscles of the Mammalia, and 

 would well agree with the fact that the red corpuscles of 

 Mammalia, in their embryonic condition, possess what have 

 previously been called nuclei, but which might perhaps more 

 properly be called nucleoli. 



In the anterior part of the blastoderm the processes, as I 

 have stated, are longer and thinner, and the spaces enclosed 

 between them are larger. This is clearly brought out in 

 PI. 2, fig. 4. But, besides these large spaces, there are 

 other smaller spaces, such as that at v. It is, on account of 

 the transparency of the protoplasm, very difficult to decide 

 whether these are vacuoles or simply spaces enclosed by the 

 processes, but I am inclined to think that they are merely 

 spaces. The difficulty of exactly determining this point is 

 increased by the presence of numerous white-yolk spherules 

 in the hypoblast above, which considerably obscure the view. 

 At about the same time that the blood-corpuscles appear in 

 the posterior end of the pellucid area, or frequently a little 

 later, they begin to be formed in the anterior part also. The 



