-16- 



GROWTH IN SIZE AND 



BODY PROPORTIONS IN 



FARM ANIMALS 



Sir John Hammond 



CAMBRIDGE UNIVERSITY 



Wide variations in size and in body proportions exist within each 

 species of farm animals. In recent years many investigations have been 

 undertaken to determine the causes of these differences and thereby 

 obtain a measure of control over them. 



While the factors affecting size and body proportions will be 

 treated separately here, a relationship exists between tlie various fac- 

 tors within a single breed, in that the larger the animal is for its age 

 the more advanced will be its proportional development. This applies 

 particularly to both sex and nutritional differences in size, examples of 

 which will be given. This relationship, however, does not necessarily 

 apply between breeds, for small breeds, in general, mature earlier in 

 their body proportions than large breeds do. This also holds within a 

 breed over a period of time: for example, a considerable reduction in 

 size has occurred over the last 50 years in our major beef breeds of 

 cattle, which have been selected for early maturation in body pro- 

 portions. 



Growth in size 



The intra-uterine period can be divided into three distinct phases, 

 in each of which the mode of nutrition is different. In the first, or blasto- 

 cyst phase, nutrition is derived from the uterine secretion. In the sec- 

 ond, or embryonic phase, nutrition is derived from the active erosive 

 agency of the fetal trophoblast, while in the third, or fetal phase, tro- 



321 



