sequential row selection in dataframe

Dear all;

I'm wondering if there is any 'efficient' approach for selecting a
sample of 'every nth rows'  from a dataframe. For example, let's use
the dataframe GAGurine in MASS library:

length(GAGurine[,1])

[1] 314

# select an 75% of the dataset, i.e. = 236 rows, every 2 rows starting
from row 1

test<-GAGurine[seq(1,314,2),]
length(test[,1])

[1] 157

# so, I still need another 79 rows, one way could be:
test2<-GAGurine[-seq(1,314,2),]

length(test2[,1])

[1] 157

test3<-test2[seq(1,157,2),]

# and then
final<-rbind(test2,test3)

length(final[,1])

[1] 236

Does anyone have a better idea to get the same results but without
creating different datasets like test2 and test3?

Thanks
PM