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ZOOLOGY: H. E. JORDAN 
gressive vascular channels. Furthermore, no atrophy of vessels nor 
any uncommonly extensive disintegration of erythrocytes have appeared 
in the yolk sac at these early stages (7 mm. in mongoose embryo; 10 
mm. pig embryo) and yet the endothelial derivation of hemoblasts is 
very active. 
It may be emphasized that as regards the endotheHal origin and the 
composition of the aortic cell clusters, and as regards the mesothelial 
origin of cellular elements of the serous fluids, Emmel and I are in es- 
sential agreement. But Emmel views these structures as the result 
of the presence of a stimulating toxin; I see in them only the expression 
of a normal inherent capacity of embryonic endothelium to produce 
blood cells. The explanation of the hmited distribution of the clusters 
is to be found in a relationship to young or newly formed, only slightly 
differentiated, endothelium; rather than in a connection with regressive 
blood vessels and an associated toxic substance. 
All the facts seem to fit better the h5^othesis that the hemogenic 
activity of embryonic endothelium is a normal function at a certain 
stage of embryonic development, than that the causative stimulus is a 
toxin derived from degenerating vascular tissues. 
1 Maximow, A., Arch. mikr. Anat., Bonn, 73, 1909, (444-561). 
2 Minot, C, Keibel, and Mall, Human Embryology, Philadelphia, 2, 1912, (498-534). 
3 Emmel, V. E., Anat. Rec, Philadelphia, 9, 1915, (77-78). 
^ Jordan, H. E., Ibid., 10, 1916, (417-420). 
5 Emmel, V. E., Amer. J. Anat., Philadelphia, 19, 1916, (401^22). 
^ Dantschakoff, V., Folia Haematologica, Leipzig, 4, 1907, (159). 
7 Stockard, C. R., Amer. J. Anat., Philadelphia, 18, 1915, (227-327). 
8 Hahn, H., Arch. Entw.-Mech., Jena, 27 j 1909, (91). 
9 Miller, A. M., and McWhorter, J. E., Anat. Rec, Philadelphia, 8, 1914, (203-227). 
10 Reagan, F. P., Ibid., 9, 1915, (329-341). 
11 Schulte, H. von W., Mem. Wistar Inst., Philadelphia, 3, 1914, (1-90). 
12 McClure, C. W. F., Ibid., 4, 1915, (1-140). 
13 Huntington, G. S., Amer. J. Anat., Philadelphia, 16, 1914, (259-316). 
14 Saline, F. R , Science, New York, N. S., 44, 1916, (145-158). 
15 The mongoose embryos were collected at Montego Bay, Jamaica, B. W. I., in March, 
1912; the loggerhead turtle embryos on Loggerhead Key, Florida, in the summer of 1914. 
I am indebted to the Department of Marine Biology of the Carnegie Institution of Wash- 
ington for the privilege of accompanying the scientific expedition to Jamaica, and for 
the excellent facilities provided at Loggerhead Key. To the Director, Dr. Alfred G. 
Mayer, I am under obligations for valuable help and many kindnesses. 
16 Jordan, H. E., Amer. J. Anat., Philadelphia, 19, 1916, (277-304). 
17 Bremer, J. L., Ibid., 16, 1914, (447-476). 
18 Emmel, V. E., Ibid., 20, 1916, (73-124). 
19 Mallory, F. B., The Principles of Pathologic Histology, Philadelphia, 1914, (1-677). 
