252 ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 



bation four zones are to be made out : an outer and an inner zone of the area opaca 

 and an outer and axial zone of the area pellucida. In the outer and the inner zones 

 of the area opaca one will always find two different generations of blood-islands; 

 if one zone has young islands the other will have an older generation. In the area 

 pellucida, during these stages, the observer can follow one generation of islands after 

 another in the vessels of the posterior arch, while the posterior axial zone in the 

 stages which we are considering continues to be an area for the differentiation of 

 new angioblasts with but small contributions to the number of red cells. 



At the stage when the omphalo-mesenteric arteries are forming, figure 13, 

 plate 3, another factor must be recognized in the living specimen, namely, that 

 along the vessels the mesenchyme cells begin to form in chains along the outer wall 

 which must be distinguished from the endothelium. These cells represent the addi- 

 tion of mesenchyme to the wall of the vessel in the process of development from a 

 capillary into an artery. They are the forerunners of the media and adventia. In 

 the living chicks of 2 days of incubation I have never observed any indication of an 

 endothelial cell leaving the wall of a vessel, a condition described for later stages by 

 Madame Danchakoff (1909) in connection with the formation of blood-cells. 



CYCLES IN CELL DIVISION. 



These cyles in the development of the vascular system are dependent on the 

 fact of cycles in cell division. In these living specimens it has become clear that, 

 in the case of three tissues at least, there are definite cycles of cell division, i. e., 

 in the nervous system, in the endoderm, and in the vascular system. I have not 

 yet studied the ectoderm in this regard, except in the nervous system, and have 

 not found the process so easy to follow in the mesoderm lining the ccelom. In the 

 case of the endoderm attention has already been called to the fact that when the 

 entire endoderm passes into the refractive state of the cytoplasm which precedes 

 cell division, the specimen can not be studied for vessels. If such a specimen is 

 fixed at once nothing in the stained preparation will indicate that the cells were 

 about to divide; but if fixation is delayed until an occasional nucleus can be seen 

 in the metaphase, it will be observed that nearly all of the nuclei in the endoderm 

 show division. I have many specimens showing this cycle of division in the endo- 

 derm and in the nervous system. The phenomenon is illustrated in connection 

 with the blood-islands in figures 18 and 19, plate 4. In such specimens as the one 

 shown in figure 18, plate 4, if one watches the living island until all of the cells have 

 divided, the entire island will appear to be of the same size as at the beginning, but 

 every cell will be half as large as it was originally. This appearance proves that all 

 of the cells divide in one cycle, notwithstanding the fact that every nucleus does not 

 show a spindle at exactly the same moment. It shows strikingly also that growth 

 (specifically increase in size) occurs in the phases between cell division. Moreover, 

 if one takes the zones of development which have just been described, namely, the 

 outer and inner zone of the area opaca and the outer and axial zone of the posterior 

 part of the area pellucida, all of the cells either of angioblasts or of blood-islands, 

 whichever happen to be present in any one of these four zones will be at the same 



