IV. THE DEVELOPMENT OF THE BLOOD-VESSELS OF 



THE CHICK 1 . 

 With Plate II. 



THE development of the first blood-vessels of the yolk-sac 

 of the chick has been investigated by a large number of ob- 

 servers, but with very discordant results. A good historical 

 resume of the subject will be found in a paper of Dr Klein 

 (liii. Band der K. Akad. der Wissensch. Wien], its last in- 

 vestigator. 



The subject is an important one in reference to the homo- 

 logies of the blood-vascular system of the vertebrata. As I 

 shall shew in the sequel (and on this point my observations 

 agree with those of Dr Klein), the blood-vessels of the chick 

 do not arise as spaces or channels between the cells of the 

 mesoblast ; on the contrary, they arise as a network formed by 

 the united processes of mesoblast-cells, and it is through these 

 processes, and not in the spaces between them, that the blood 

 flows. It is, perhaps, doubtful whether a system of vessels 

 arising in this way can be considered homologous with any 

 vascular system which takes its origin from channels hollowed 

 out in between the cells of the mesoblast. 



My own researches chiefly refer to the development of the 

 blood-vessels in the pellucid area. I have worked but very 

 slightly at their development in the vascular area ; but, as far 

 as my observations go, they tend to prove that the mode of 

 their origin is the same, both for the pellucid and the vascular 

 area. 



The method which I have principally pursued has been to 

 examine the blastoderm from the under surface. It is very 

 difficult to obtain exact notions of the mode of development of 



1 From the Quarterly Journal of Microscopical Science^ Vol. XIII, 1873. 



