DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK. 53 



which the corpuscles were supposed to be developed, and to 

 escape by the rupture of the walls into the capillary spaces 

 between. This mistake has been clearly pointed out by Klein 

 (loc. tit.}. 



At the time when these spaces are formed, and especially 

 in the hinder two-thirds of the pellucid area, and in the layer 

 of blood-vessels immediately above the hypoblast, a formation 

 takes place which forms in appearance a secondary investment 

 of the capillaries. Dr Klein was the first to give a correct ac- 

 count of this formation. It results from the cells of the meso- 

 blast in the meshes of the capillary system. Certain of these 

 cells become flattened, and send out fine protoplasmic processes. 

 They arrange themselves so as completely to enclose the spaces 

 between the capillaries, forming in this way vesicles. 



Where seen on section (vide fig. 6) at the edge of the vesicles 

 these cells lining the vesicles appear spindle-shaped, and look 

 like a secondary investment of the capillaries. This investment 

 is most noticeable in the hinder two-thirds of the pellucid area ; 

 but, though less conspicuous, there is a similar formation in its 

 anterior third, where there would seem to be only veins present. 

 Dr Klein (loc. cit., fig. 12) has also drawn this investment in the 

 anterior third of the pellucid area. He has stated that the 

 vessels in the mesoblast between the splanchnopleure and the 

 somatopleure, and which are enclosed by prolongations from the 

 former, do not possess this secondary investment ; he has also 

 stated that the same is true for the sinus terminalis ; but I am 

 rather doubtful whether the generalisation will hold, that veins 

 and arteries can from the first be distinguished by the latter 

 possessing this investment. I am also rather doubtful whether 

 the spaces enclosed by the protoplasmic threads between the 

 splanchnopleure and somatopleure are the centres of vessels at 

 all, since I have never seen any blood-corpuscles in them. 



It is not easy to learn from sections much about the first 

 stages in the formation of the capillaries, and it is impossible 

 to distinguish between a completely-formed vessel and a mere 

 spherical space. The fine protoplasmic processes which connect 

 the masses of corpuscles can rarely be seen in sections, except 

 when they pass vertically, as they do occasionally (vide PI. 2, 

 fig. 9) in the opaque area, joining the somatopleure and the 



