TISSUE CULTURE 351 



Human blood is best drawn from the median basilic vein of the 

 arm. Fucns {Arch. f. exper. Zellforsch., xiv, 1933, p. 334) has 

 described a special centrifuge with which one can prepare mam- 

 malian plasma of all species without an anti-coagulant. It is not 

 necessary to balance the buckets in this improved model and the 

 speed attained throws down all the platelets in a very short tinie. 

 For further methods of preparing mammalian plasma without 

 anti-coagulants see Baroni, Compt. rend. Soc. Biol., civ, 1930, 

 p. 599. 



For the p^ determination in blood and plasma, see the methods 

 of Dale and Evans, Journ. Physiol., liv, 1920, p. 167, and Martin 

 and Lepper, Biochem. Journ., xx, 1926, p. 37. 



Preparation of Serum. Mammalian plasma, clotted in a tube 

 by the addition of a few drops of embryonic extract, readily yields 

 serum after centrifuging, if the clot is broken up with a glass rod. 

 Fowl plasma treated in this manner gives much less, but the yield 

 is increased by shaking the clotted plasma in a flask with sterile 

 glass beads and then centrifuging. 



784. Embryonic and Tissue Extracts. Chick embryos of 

 eight to ten days' incubation or mammalian embryos of approxi- 

 mately half the normal gestation period are the most suitable for 

 making nutrient extracts. In the case of chick embryos the larger 

 blunt end of the egg is tapped with forceps and the shell, forming 

 the wall of the air chamber, removed without breaking the inner 

 shell membrane. This membrane is then torn through with sterile 

 forceps, the membranes and yolk sac are severed with scissors, 

 and the embryo lifted out without touching the shell. Place the 

 embryos in a sterile Petri dish with excess Tyrode solution and 

 remove the fragments of membranes which tend to float in the 

 extract after centrifuging. Some workers also remove the eyes to 

 prevent contamination of the extract with pigment. After 

 washing the embryos in two or three changes of Tyrode to free 

 them as far as possible from blood corpuscles, transfer them to a 

 dry Petri dish and mince them finely with curved scissors. The 

 embryonic pulp should be taken up in short lengths of narrow-bore 

 glass tubing, fitted with rubber teats for transference to the 

 centrifuge tubes. Centrifuge the pulp for ten minutes at 3,000 

 revolutions per minute. This should be sufficient to throw down 

 all embryonic cells, but as an extra precaution, the extract may 

 be frozen solid with COg, or in a freezing mixture. 



Fischer {Archiv. f. exper. Zellforsch., i, 1925, p. 122) has 

 figured a simple apparatus for making tissue extracts which is very 

 useful where large quantities are required. The adaptation of a 

 glass syringe for this purpose is described by Earle {Archiv. f. 

 exper. Zellforsch., xiii, 1932, ]). 510). 



The Concentrations of Embryonic Extract. The preparation of 

 embryonic extract cannot be standardised very accurately, but 



