IV.] THE BLOOD-VESSELS. 69 



corpuscles too are being formed in considerable numbers. 

 The mottled yellow vascular area becomes covered with red 

 patches consisting of aggregations of blood-corpuscles, often 

 spoken of as blood-islands. 



Round the extreme margin of the vascular area and nearly 

 completely encircling it, is seen a thin red line, the sinus or 

 vena terminalis (Fig. 23, Sv.). This will soon increase in 

 size and importance. 



From the vascular and pellucid area several large channels 

 are seen to unite and form two large trunks, one on either 

 side, which running along the splanchnopleure 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 omphalo-mesaraic veins (Figs. 14, o.f., 23, o.f.) spoken of 

 above. 



Both vessels and corpuscles are formed entirely from the 

 cells of the mesoblast; and in the regions where the meso- 

 blast is cleft, are at first observed exclusively in the splanch- 

 nopleure. Ultimately of course they are found in the meso- 

 blast everywhere. 



The mode of formation of the blood-vessels and corpuscles has been much 

 and long debated. The observations of one of us have led us to beheve the 

 following to be the true account. 



In the pellucid area, where the formation of blood-vessels may be most 

 easily observed, a number of mesoblastic cells are seen to send out processes. 

 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 con- 

 tinued division of the nuclei, increase rapidly in size; the majority of the nuclei 

 composing them acquire a red colour and become converted into blood-corpuscles 

 (Fig. 19, b.c.) ; but a few, generally on the outside of the group, remain un- 

 unaltered, (Fig. 19, a). The protoplasm in which the central reddened nuclei 

 are imbedded becomes hquefied, while that on the outside of each group, as well 

 as that of the uniting processes, remains granular, and increasing in quantity, 

 forms an investment for the unaltered nuclei which are embedded in it. 



Each nodal point is thus transformed into a more or less rounded mass of 

 blood-corpuscles floating in plasma but enveloped by a layer of nucleated proto- 

 plasm, 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 a transformation of nuclei similar to that which took place in the nodal 

 points, blood-corpuscles make their appearance in the processes also, the central 

 portions of which become at the same time liquefied. The uncoloured nuclei 



