DEVELOPMENT OF VESSELS WITHOUT HEART 201 



at certain points, break up, and retract, the blood cells are forced 

 into little lakelets of blood surrounded by the endothelium of the 

 retracted capillary which encloses the blood cells as a capsule. 



This process continues progressively out over the area pellu- 

 cida and area opaca, but does not advance so far as in the area 

 pellucida. It manifests itself here by the narrowing of most of 

 the vessels, and the solidification and retraction of some of them 

 — processes which cause an increasing difficulty of injection. By 

 this time, the sinus terminalis is also beginning to offer resistance 

 to the injection mass. As this vessel is usually injected with 

 ease in younger chicks, it is plain that it also under goes a proc- 

 ess of narrowing which precedes its breaking-up into capil- 

 laries. The blood within these small vessels is contained under 

 some pressure. This can be demonstrated by puncturing them 

 with the injection needle which is usually sufficient to produce an 

 extravasation of blood cells. 



The general application of the histo-mechanical laws of Thoma 

 to the factors observed in this experiment are too obvious to permit 

 of much discussion. The failure of the omphalo-mesenteric ar- 

 teries and vein to develop show that these vessels are entirely 

 dependent upon the blood-stream for their development. Thoma 

 observed this factor and studies the development of these vessels. 

 He states that with the beginning of the circulation, a few chan- 

 nels in the capillary net are selected by the blood-stream in conse- 

 quence of the general direction which is given to it by the posi- 

 tion of the ends of the primitive aorta on the one side and of the 

 venous ostia of the heart on the other. These channels contain 

 the more rapidly flowing streams. They, therefore, dilate and be- 

 come converted into arteries and veins. Thoma, did not, how- 

 ever, recognize the formation of any arteries or veins in the extra- 

 embryonic region as due to other than mechanical forces. 



The response of the capillaries to the feeble circulation main- 

 tained by the pulsating vesicles in the extra-embryonic region 

 is another factor in support of his description of the formation 

 of larger vessels from the plexus by a circulation. 



Thoma states further that, after the beginning of the circula- 

 tion, some channels which offer resistance to the flow of the blood, 



