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CELLS, TISSUES, AND ORGANISMS 



Figure 3. The effect of the mother's size on the size of the foal in reciprocal crosses 

 between the large Shire horse and the small Shetland pony. At the left are the parents 

 and the foal (at birth and at one month) representing the mating of a Shire stallion and 

 a Shetland mare; at the right is the larger foal of a Shire mare and a Shetland stallion. 

 (Walton and Hammond, 1938.) 



birth. This has been shown by reciprocal crosses between large and 

 small breeds in horses (Walton & Hammond, 1938), in which the 

 weight of the placenta, and of the foal, of a large mother is three times 

 that of a small mother ( Figure 3 ) . This has been also shown for cattle 

 (Hammond and Joubert, 1958) and sheep (Hunter, 1956). The ge- 

 netics of the sire determine the upper limit of size at birth in the large 

 mother, but the size of the placenta limits it in the small mother. That 

 this is due to maternal nutrition and not to cytoplasmic inheritance 

 is shown by the fact that fertilized eggs reciprocally transplanted 

 between large and small breeds of sheep still show these effects 

 ( Figure 4 ) . 



The length of time that these maternal influences on size persist is 

 determined by the stage of development at which the young are born. 

 In the horse, where the leg has reached full length (from knee to 

 ground) at birth, differences in birth size persist into adult life (Wal- 

 ton and Hammond, 1938 ) . This can also be seen in the size differences 

 between mules and hinnies. But in sheep, the length of whose cannon 

 bone is not fully developed at birth, maternal effects on size diminish 



