ERYTHROBLASTS IN THE PIG EMBRYO 161 



a. Plasma, temperature and other conditions 



The necessity of maintaming the blood cells studied in as 

 normal a medium as possible need not be emphasized. It has 

 already been indicated that the plasma used was from the same 

 embryo whose blood cells were being investigated unless other- 

 wise stated. Since the quantity of blood in 25 to 35 mm. embryos 

 is too small to be favorable for centrifuging, the blood fluid was 

 taken directly from the heart. Through the fact that the per- 

 centage of blood cells in a given volume of the plasma is much 

 less in these embryos than in older specimens or adults, together 

 with the tendency of the cells to settle to a lower level, plasma 

 can be obtained which contains a minimum amount of corpuscles. 

 Precautions were taken to obtain this plasma with as little mod- 

 ification as possible. Before removing the embryo from the uterus, 

 the umbilical cord was clamped with a serrefine forceps so as to 

 guard against the possible entrance of foreign fluids into the cut 

 umbilical veins through the suction produced by the beating heart. 

 The cord was then cut and the embryo removed to a sterilized 

 towel, a pad of cotton, or a sheet of filter paper. The amniotic 

 fluid was washed from the embryo by several rinsings with ster- 

 iUzed Ringer's solution (39°), and this solution in turn removed 

 with sterilized filter paper. The heart was then exposed by cut- 

 ting away part of the thoracic wall and pericardium. Filter paper 

 was again employed to remove the small amount of pleural and 

 pericardial fluids. The heart while still beating was then opened 

 and a small quantity of blood quickly transferred to carefully 

 cleaned, sterile cover-glasses. For this transference several 

 methods were tried including the use of glass rods, glass pipettes 

 and platinum wire loops. The best results were obtained, how- 

 ever, with a small wedge of cardiac tissue, a millimeter or two in 

 length, cut out from the heart wall itself. This piece of tissue 

 was seized with a very fine forceps, immersed in the heart blood 

 and the adhering quantity of plasma and corpuscles brought 

 to the cover-glass. Since the blood corpuscles which are thus 

 transferred with the plasma are sufficiently reduced in number 

 to be favorable for culture purposes, the cover-glass can be at 



