52 DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK. 



the protoplasmic network, a large number of the connecting 

 processes in this network have become filled with blood-cor- 

 puscles. The appearances presented by the network at a 

 slightly later stage than this is shewn in PL 2, fig. 6, but in 

 this figure all the processes are seen to be filled with blood- 

 corpuscles. 



This investment of the masses of corpuscles by a cellular 

 wall occurs much earlier in some specimens than in others, both 

 in relation to the time of incubation and to the completion of 

 the network. It is generally completed in some parts by the 

 time there are eight or nine proto-vertebrae, and is almost 

 always formed over a great part of the pellucid area by the 

 thirty-sixth hour. The formation of the corpuscles, as was 

 pointed out above, occurs earliest in the central part of the 

 hour-glass shaped pellucid area, and latest in its anterior part. 

 In the hinder part of the pellucid area the processes, as well 

 as their enlarged starting-points, become entirely filled with 

 corpuscles ; this, however, is by no means the case in its an- 

 terior part. Here, although the corpuscles are undoubtedly 

 developed in parts as shewn in fig. 7, yet a large number of 

 the .processes are entirely without them. Their development, 

 moreover, is in many cases very much later. When the de- 

 velopment has reached the stage described, very little is re- 

 quired to complete the capillary system. There are always, of 

 course, a certain number of the processes which end blindly, 

 and others are late in their development, and are not by this 

 time opened ; but, as a general rule, when the cellular invest- 

 ment is formed for the masses of corpuscles, there is completed 

 an open network of tubes with cellular walls, which are more or 

 less filled with corpuscles. These become quickly driven into 

 the opaque area in which at that time more corpuscles may 

 almost always be seen than in the pellucid area. 



By the formation of a network of this kind it is clear that 

 there must result spaces enclosed between the walls of the 

 capillaries ; these spaces have under the microscope somewhat 

 the appearance of being vesicles enclosed by walls formed of 

 spindle-shaped cells. In reality they are only spaces enclosed 

 at the sides, and, as a general rule, not above and below. 

 They have been mistaken by some observers for vesicles in 



