242 Experimental Zoology 



These results are for higher animals. In the lower animals, 

 where the percentage of water is often very great, the results 

 may be somewhat different, but we lack data at present on this 

 point. The amount of water in adult medusae is about 95 per 

 cent; in sea anemones, 87; in sponges, 74 to 84; in the earth- 

 worm, 87 ; in the slug, 87 ; in one of the ascidians, Botryllus, 

 93 ; in the crayfish, 71 per cent. In a chick twenty-one days old, 

 i.e. at the time of hatching, the water present is 80 per cent. In 

 man there is 66 per cent of water; but this varies greatly in 

 different parts of the body : thus, in the enamel, 2 per cent ; in the 

 bones, 22 ; in the muscles, 75 ; in the blood, 79 per cent. 



The most complete account that we have at present of the 

 growth of an animal from birth to maturity is that by Minot 

 in his paper entitled "Senescence and Rejuvenation." I shall 

 give, therefore, a detailed account of his results. Minot used 

 guinea pigs for his work. The number of young born in differ- 

 ent litters is given in the following table : — 



No. in a litter 12345678 



No. of litters observed 23 58 37 182 2 2 i 



It will be seen that htters of two are most frequent, correspond- 

 ing with the number of mammae of the mother. Yet there is no 

 close correspondence, since over half the litters contain more than 

 two young. What the conditions are that determine the num- 

 ber of young in a litter is not entirely clear ; one fact at least was 

 made out, viz. that older mothers had larger litters. This is 

 shown in the following table : ^ — 



No. in a litter .... i 23 4 5678 



Average age of mothers . 200.9 286 289.7 464.8 104 ? 200 433 days 

 No. of observations . . 18 5127 15 i o i i 



Minot thinks that there is also an individual tendency for 

 certain individuals to produce litters of a definite size, which is 

 probably due to a tendency to set free from the ovary fewer or 

 more eggs. For instance, two cases of successive litters gave: 



' Litters of 5, 7, and 8 are based on a single observation, and Minot says have 

 little value. 



