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 medusz 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; inthe crayfish, 71 percent. Ina chick twenty-one days old, 
i.e. at the time of hatching, the water present is 80 percent. 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. : : : . ‘ 5 I2 345678 
No. of litters observed ‘ . : ; 23 58 37 182 221 
It will be seen that litters of two are most frequent, correspond- 
ing with the number of mamme 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. inalitter . . . . I 2 33 4 56 7 8 
Average age of mothers 200.9 286 289.7 464.8 104 ? 200 433 days 
No. of observations . . 18 5I 27 15 iro. © 2 
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. 
