Development, &e., of Blood-vessels. By G. and F. E. Eoggan. 573 



the actual condition, as will afterwards appear. For convenience' 

 sake, let us commence with Fig. 1, where we see a spindle-shaped 

 or bipolar cell, a, forming a junction with a comparatively large 

 blood-vessel h. We speak of cell a as bipolar, because it has 

 stretched out one-half of its protoplasm peripherally on one side, 

 while the other half is stretched out centrally in connection with the 

 blood-vessel. We may also note here that whatever shape a 

 wandering cell may possess before it makes up its mind to enter 

 into the composition of a blood-vessel, no sooner is that settled than 

 it assumes the elongated form, being either unipolar or bipolar from 

 the nucleus, and in no case yet have we seen a many-branched or 

 stellate cell entering into connection with a developing blood-vessel. 

 In Fig. 1 we also notice that the wall of the blood-vessel is bulged 

 out at its attachment to the protoplasm of the peripheral vessel- 

 forming cell, a, and the question is an open one whether the bulging 

 is the result of sympathy on the part of the vessel towards the cell 

 a approaching from the periphery, or whether the bulging merely 

 marks the spot where the cell a actually passed through the vessel- 

 wall from its interior foiuards the periphery to take its place where 

 it is now seen. 



Passing on from Fig. 1, let us follow up the process in Fig. 2, 

 which takes us on a stage further in the same direction. Here we 

 have a cell, a, also lying at right angles to the existing vessel, in 

 this case a capillary, which has not merely bulged, but actually 

 bent its tube towards cell a. This cell may also be called bipolar, 

 although the great bulk of its protoplasm lies between the capillary 

 and the nucleus, which latter lies twice as far from the capillary as in 

 Fig. 1, or, if we consider the direction of movement to be reversed, 

 it has passed twice as far from the vessel as cell a, Fig. 1. We 

 see here also two points of interest already referred to as illustrating 

 fixed laws throughout : — 1st, The point of the attached cell is not 

 applied directly at right angles to the vessel, but lies alongside of it 

 (as was also the case in Fig. 1 ), and in the second place, at the 

 weakest part of the joint thus formed, we see the cell c placing 

 itself so as to strengthen the newly-formed joint or splice. Let us 

 next pass to a more advanced stage, as seen in i, Fig. 12, where 

 the cell is applied to a formed vessel, as in Figs. 1 and 2. Here 

 we have the joint strengthened b}' the application of another cell, as 

 in Fig. 2, and we have the first traces of another important feature, 

 the formation of the vacuole, or hollowing out of the cell whose 

 protoplasm is to form the capillary wall. It is to be seen as a small 

 white spot lying close to the nucleus on the side next the capillary, 

 and will go on increasing in size until either directly or by the 

 medium of an additional cell it forms a connection with the cavity 

 of the formed vessel. 



A fourth stage is scon in Fig. 3, where not only has the cavity 



