Eben C. Hill 441 
the youngest stages, measuring from eighteen min. to seventy-five mm., a 
hypodermic syringe with a fine needle forced the injection fluid into one 
of the umbilical arteries, and by watching the hind legs and head excellent 
results devoid of extravasation could be obtained. This mode of in- 
jection is particularly desirable in these early stages for it is not nec- 
essary to rupture the surrounding membranes and thus the embryo is 
protected against injury in handling. In larger embryos injections were 
made directly into the aorta by puncturing the left ventricle. Consid- 
erable pressure was necessary to overcome in the earlier stages the re- 
sistance resulting from the small lumen of the spermatic artery, and in 
larger embryos because of its remarkable tortuosity. On account of this 
pressure the Wolffian bodies were frequently doubly injected, the in- 
jection mass passing through the sinusoids and capillaries described by 
Minot into the veins. In nearly all such specimens the testes showed 
only an arterial and capillary injection. In this connection it may not 
be amiss to emphasize the advantages of India ink in all cases where 
a fluid is desired which will flow wherever the blood stream goes, and 
yet is resistant to the ordinary laboratory acids and to concentrated 
solutions of potassium or sodium hydroxide. 
After the injection of each embryo, the right testis together with the 
Wolffian body, kidney, and aorta were removed and placed in ninety-five 
per cent alcohol for clearing, while the left sex gland with its appendages 
was prepared for sectioning. Of the various clearing procedures, the 
modified Schultze method* was found to give the most satisfactory 
results. This method is as follows: 
After the injections have been completed all unnecessary tissues sur- 
rounding the parts under investigation are removed and the specimens 
are placed in ninety-five per cent alcohol. The removal of adventitial 
tissue is most important, though entire embryos may be successfully 
cleared if openings are made into the abdomen, thorax and cranium. In 
embryos ranging above one hundred and fifty mm. in size, it is still 
better to make sagittal sections of the hardened specimens and to clear 
in halves. The alcohol should be frequently changed and large quanti- 
ties should be used. In order to obtain transparent specimens the 
tissues must be completely shrivelled before removal from the alcohol, 
and the length of time necessary to accomplish this result depends, of 
course, upon the size of the objects. For very small specimens at least 
4Hill, E. C.: On the Schultze Clearing Method as Used in the Anatomical 
Laboratory of the Johns Hopkins University. Johns Hop. Hosp. Bull., Vol. 
XVII. 
