54 DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK. 



splanchnopleure. Dr Klein considers these latter processes to 

 be the walls of the vessels, but they appear rather to be the 

 processes which will eventually become new capillaries. 



From sections, however, it is easy to see that the appear- 

 ances of the capillaries in the vascular area are similar to the 

 appearances in the pellucid area, from which it is fair to con- 

 clude that their mode of formation is the same in both. It is 

 also easy to see that the first formation of vessels occurs in the 

 splanchnopleure, and that even up to the forty-fifth hour but few 

 or no vessels are found in the somatopleure. The mesoblast of 

 the somatopleure is continued into the opaque area as a single 

 layer of spindle-shaped cells. 



Sections clearly shew in the case of most of the vessels that 

 the secondary investment of Klein is present, even in the case of 

 those vessels which lie immediately under the somatopleure. 



In reference to the origin of particular vessels I have not 

 much to say. Dr Klein's account of the origin of the sinus 

 terminalis is quite correct. It arises by a number of the 

 masses of blood-corpuscles, similar to those described above, 

 becoming connected together by protoplasmic processes. The 

 whole is subsequently converted into a continuous vessel in the 

 .usual way. 



From the first the sinus terminalis possesses cellular walls, 

 as is clear from its mode of origin. I am inclined to think 

 that Klein is right in saying that the aortae arise in a similar 

 manner, but I have not worked out their mode of origin very 

 fully. 



It will be seen from the account given above that, in refer- 

 ence to the first stages in the development of the blood-vessels, 

 my observations differ very considerably from those of Dr Klein ; 

 as to the later stages, however, we are in tolerable agreement. 

 We are in agreement, moreover, as to the fundamental fact that 

 the blood-vessels are formed by a number of cells becoming 

 connected, and by a series of changes converted into a network 

 of vessels, and that they are not in the first instance merely 

 channels between the cells of the mesoblast. 



By the forty-fifth hour colourless corpuscles are to be found 

 in the blood whose exact origin I could not determine ; pro- 

 bably they come from the walls of the capillaries. 



