92 THE SECOND DAY. [CHAP. 



one on either side, which running along the splanch- 

 nopleure folds at nearly right angles to the axis of the 

 embryo, unite at the " point of divergence " to join the 

 venous end of the heart. These are the vitelline veins 

 spoken of above. 



Both vessels and corpuscles are formed entirely 

 from the cells of the mesoblast; arid in the regions 

 where the mesoblast is cleft, are at first observed ex- 

 clusively in the splanchnopleure. Ultimately of course 

 they are found in the mesoblast everywhere. 



In the pellucid area, where the formation of the blood-vessels 

 may be most easily observed, a number of mesoblastic cells are 

 seen to send out processes (Fig. 33). These processes unite, and 

 by their union a protoplasmic network is formed containing 

 nuclei at the points from which the processes started. The 

 nuclei, which as a rule are much elongated and contain large oval 

 nucleoli, increase very rapidly by division, and thus form groups 

 of nuclei at the, so to speak, nodal points of the network. 

 Several nuclei may also be seen here and there in the processes 

 themselves. The network being completed, these groups, by 

 continued division of the nuclei, increase rapidly in size ; the 

 protoplasm around them acquires a red colour, and the whole 

 mass breaks up into blood-corpuscles (Fig. 33, b.c.) The proto- 

 plasm on the outside of each group, as well as that of the uniting 

 processes, remains granular, and together with the nuclei in it 

 forms the walls of the blood-vessels. A plasma is secreted by 

 the walls, and in this the blood-corpuscles float freely. 



Each nodal point is thus transformed into a more or less 

 rounded mass of blood-corpuscles floating in plasma but en- 

 veloped by a layer of nucleated protoplasm, the several groups 

 being united by strands of nucleated protoplasm. These uniting 

 strands rapidly increase in thickness; new processes are also 

 continually being formed ; and thus the network is kept close 

 and thickset while the area is increasing in size. 



By changes similar to those which took place in the nodal 



