5o6 Remarks on the Morphology of Arteries^ [May, 



their contour or by means of processes which they throw out, so 

 that there results a net-work, at first indistinct but gradually in- 

 creasing in clearness and color as the cells enlarge. 



Each of these blood-islands then undergoes vacuolation, a por- 

 tion of the protoplasmic contents becoming liquified and leaving 

 a cavity. By a budding process new cells are formed in the in- 

 terior of the mother-cells and becoming detached float free in the 

 fluid which fills the vacuole. At this period, therefore, the blood- 

 islands present an outside cell-wall with a contained fluid in 

 which float free cells or corpuscles, the whole arranged in a close 

 mesh-work. 



Next the cell-walls wherever in contact thin away and disap- 

 pear, there resulting a tube the walls of which are the original 

 cell-walls of the blood-islands, the contents a fluid, plasma, in 

 which swim free blood-corpuscles. There is at first no special 

 difference in size among the vessels thus formed, nor is there any 

 structural difference by which we can distinguish arteries from 

 veins. No trunks or branches can as yet be made out, it is in 

 fact a capillary plexus that appears, all vessels lying on the same 

 plane and communicating equally with each other. 



But a difference soon begins to be manifest. The rapidity of 

 growth varies greatly. Along certain lines the vessels begin to 

 increase in size so that soon there is visible distinction of capil- 

 laries, branches and trunks. This process of capillary and trunk 

 formation extends from without inward, attains the proper body 

 of the embryo, finally reaching the rudimentary vesicle which 

 represents the heart. (Plate VIII.) 



It should be noted that the development is centripetal. Noth- 

 ing is more natural than to look upon the arteries as a system 

 proceeding centrifugally from the heart outward. 



However convenient this may be to the physiologist or the 

 surgeon, to the anatomist it embodies a fallacy. The capillaries 

 are the first formed, next the arterioles, then the branches of 

 larger size, finally the trunks. It is owing to the subtle persistence 

 of this fallac}^ that the study of the arterial system has advanced 

 no farther. 



I have mentioned that the rapidity of growth is greater along 

 certain lines, thus leading to the formation of trunks. It is con- 

 ceivable that these trunk-lines should be intermediate in direc- 

 tion, but in fact they usually become established in certain definite 

 situations. What can be ascertained as to the causes for this ? 



