ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 241 



It is this exact observation that has now been made in the blastoderm of a 

 chick by the application of the method of tissue-culture. One can actually see the 

 division of an endothelial cell into two daughter-cells, one of which remains in the 

 wall of the vessel as a part of its endothelial lining, while the other protrudes into 

 the lumen and becomes a unicellular island, the forerunner of a mass of erythro- 

 blasts. That endothelium gives rise to erythroblasts may therefore be accepted 

 as proved in the case of the chick. The studies herein recorded do not include any 

 observations of the origin of the white blood-cells, since there are no cells in the 

 chick of the second day incubation that can be identified as the forerunners of the 

 white cells, all of the cells in the islands and free in the vessels having hemoglobin. 



In 1892 Schmidt described the origin of both red and white blood-cells from 

 the endothelium of the vessels of the liver and spleen in human embryos. He 

 described localized areas of mitosis in the endothelium of the capillaries of the 

 human liver during development and interpreted them correctly as giving rise to 

 clumps of blood-cells. These he interpreted as both red and white cells. In his own 

 words he concludes (p. 220) : 



"In der embryonalen Leber findet eine mit der Gefassentwicklung im Zusammenhang 

 stehende Neubildung weisser und rother Blutkorperchen statt. Die ersteren werden von 

 den Endothelien der Capillaren durch karyokinetische Theilung producirt und pflanzen sich 

 selbst durch Mitose welter fort. Die rothen entstehen aus den farblosen durch Auftreten 

 von Haemoglobin im Protoplasma und besitzen ebenfalls die Fiihigkeit aquivalenter 

 Theilung durch Mitose." 



Since that time many authors have given evidences of the origin of blood- 

 cells from endothelium, both in birds and in mammals, as will be described later 

 in connection with the origin of blood-cells from the endothelium of the aorta. 



CYCLES IN THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



It will be interesting to take up what I shall call the cycles in the development 

 of the vascular system, and which I shall subsequently show are due to successive 

 cycles in cell division. This subject is illustrated in a series of photographs, 

 plates 1 to 3. At the stage shown in figure 2, plate 1, when there is no head-fold, 

 the mesoderm is almost undifferentiated. In the area pellucida there are two 

 zones of mesoderm, an axial, dense mass, and a lateral, less dense zone. In the 

 lateral part there is little indication of the zones which will ultimately divide into 

 three parts; an anterior zone of the amino-cardiac vesicles, a middle zone which 

 will ultimately lie opposite the venous end of the heart, and in which the vitelline 

 veins will develop, and a posterior zone in which the omphalo-mesenteric arteries 

 will appear. Over the area opaca the mesoderm is dense, and over the posterior 

 area, especially, it is much mottled. It has already been brought out that the mass 

 of mesoderm at this stage is but slightly differentiated into the cells that go to make 

 up the exoccelom and those that become angioblasts. In the figure of a slightly 

 older stage given by Ruckert (1906, fig. 880, p. 1210) can be seen the differentiation 

 between angioblasts and the exocoelom, the faint rings around the dark spots repre- 

 senting vesicles of the exocoelom beneath, that is dorsal to the angioblasts, if I inter- 

 pret the drawing correctly. 



