48 CHARLES E. STOCKARD 



('02) would have one believe. It is probably true that in the 

 trout family also, wandering mesenchymal cells are of great 

 importance in the formation of extra-embryonic vascular endo- 

 thelium. There is a strong possibility, as admitted by Swaen 

 and Brachet ('01) that some blood cells are also formed on the 

 yolk-sac of the trout from wandering mesenchymal cells. 



The blood cells in the yolk islands increase by mitotic divi- 

 sion and soon become very prominent features in those embryos 

 without a circulation, so that in old individuals of eight or ten 

 days the entire posterior and ventral regions of the yolk are 

 almost completely covered with red blood islands. The cor- 

 puscles in these blood islands persist in a more or less normal 

 condition for a considerable length of time. 



4. Fate of the blood corpuscles in embryos without a circulation 



Figure 42 shows the corpuscles in a yolk-vessel of an embryo 

 of sixteen days old in which the blood had never circulated. The 

 vascular endothelium is well formed about the corpuscles and 

 proliferation, or multiplication of blood cells has completely 

 ceased, the nuclei are very densely stained and somewhat pyc- 

 notic in appearance suggesting a more or less atrophic condition 

 of these erythrocytes. 



Figure 40 is a cross-section of a vessel from the yolk-sac of 

 a normal embryo of seven days old. In this the vascular endo- 

 thelium is also fully developed, large chromatophores have 

 spread themselves along the vessel wall and the erythrocytes 

 are in a vigorous physiological state. The nuclei are lightly 

 staining alveolar structures quite different in appearance from 

 those of the erythrocytes in the older embryo of sixteen days 

 that has never had its blood to circulate. Yet the erythrocytes 

 in the old non-circulating embryo are ladened with haemo- 

 globin and certainly function to some degree. 



Figure 41 in the same group is a section through the inter- 

 mediate cell mass of a sixteen-day embryo without a blood 

 circulation. This is the only intra-embryonic vessel which 

 contains blood cells in this individual. The vascular endothelium 



