60 H. D. SENIOR 
b. Material 
In the present investigation the lower extremities of embryos 
have been studied in seven stages of development, one extremity 
or both having been reconstructed in wax. The selection of 
stages depended upon the accessibility of well-preserved material 
rather than upon a preconceived plan regarding the most in- 
structive stages to use.” 
The following embryos have been studied, the right limb 
having been reconstructed in all cases. The embryos of which 
both lower limbs have been reconstructed are marked with an 
asterisk : 
6.0 mm.* Carnegie Institution, Embryological Collection (C.I.E.C.) No. 
1075. 
8.5 mm.* Cornell University, Embryological Collection (C.E.C.) No. 9. 
12.0 mm.* Cornell University, Embryological Collection (C.E.C.) No. 3. 
12.0 mm.* Minnesota, Embryological Collection (M.E.C.) No. H. 16. 
14.0 mm. Cornell University, Embryological Collection (C.E.C.) No. 5. 
17.8 (?) Harvard University, Embryological Collection (H.E.C.) No. 839. 
18.0mm. Carnegie Institution, Embryological Collection (C.I.E.C.) No. 409. 
22.0 mm. Cornell University, Embryological Collection (C.E.C.) No. 1. 
For their generosity in placing their material at my disposal, 
I wish to express my great obligation to Profs. C. M. Jackson, 
F. T. Lewis, F. P. Mall, C. R. Stockard, and G. L. Streeter. 
Toward the end of the investigation, there were a number of 
difficulties, for the solution of which the examination of rather 
close intermediate stages was required. Such stages were found 
in the collection of the Carnegie Institution at Baltimore. 
In the formation of this valuable collection, so ably cared for 
and sympathetically administered, Professor Mall has made 
available to his fellow-workers a wealth of material adequate for 
the solution of any ordinary problem in human embryology. 
The reconstruction of vascular plexuses in wax, using every 
second or fourth section as the case may be, is somewhat difficult. 
The practice followed has been to unite the parts of the adjacent 
2 In the case of the embryos from the Carnegie Institution and Minnesota 
University the measurement is crown-rump. The Harvard and Cornell measure- 
ments represent the greatest total length. 
