From DATASET:
'data.frame':   578 obs. of  4 variables:
 $ weight: int  42 51 59 64 76 93 106 125 149 171 ...
 $ Time  : int  0 4 8 12 16 20 24 28 32 36 ...
 $ Chick : int  1 1 1 1 1 1 1 1 1 1 ...
 $ Diet  : int  1 1 1 1 1 1 1 1 1 1 ...

From ChickWeight:
Classes ‘nfnGroupedData’, ‘nfGroupedData’, ‘groupedData’ and 'data.frame':  578 obs. of  4 variables:
 $ weight: num  42 51 59 64 76 93 106 125 149 171 ...
 $ Time  : num  0 2 4 6 8 10 12 14 16 18 ...
 $ Chick : Ord.factor w/ 50 levels "18"<"16"<"15"<..: 15 15 15 15 15 15 15 15 15 15 ...
 $ Diet  : Factor w/ 4 levels "1","2","3","4": 1 1 1 1 1 1 1 1 1 1 ...
 - attr(*, "formula")=Class 'formula'  language weight ~ Time | Chick....etc.