The Concise Oxford Dictionary of World Religions (2041 page)

, and are usually performed by the casting of milk,
gh
(
gh
ta
), grain, etc., into the fire. In the later Vedic period, sacrifices became elaborately detailed, and a distinction was made between
g
hya
sacrifices (which rested on
sm
rta
, i.e. oral tradition and memory) and
rauta sacrifices (those based on
ruti
). The
S
ma Veda
and
Yajur Veda
were composed for the purposes of sacrifice, and the Br
hma
as were compiled with a major purpose of explaining the meaning of the sacrifices. Whereas in the earlier sacrifices there had been a strong element of
do ut des
(see introductory paragraph), there now developed a sense that the gods were dependent on sacrifices and to an extent under the control of humans (or more specifically, of priests). The
rauta sacrifices are traditionally divided into two groups of seven, Haviryajñas (including
Agnihotra
, animal sacrifices, and Pi
apit
yajña) and Somayajñas. Four groups of priests were required (headed by four chief priests):

(i) Hot
, who invokes the gods by reciting verses from the
g Veda
;
(ii) Udg
t
, the chanter of
s
mans
;
(iii) Adhvaryu, the performer of the sacrifice;
(iv) the
brahman
, who supervises the whole procedure, making sure that no errors are made.