CHAP. XII The Life-History of Animals iqi 



simply unfolded. It was to this unfolding that the word 

 evolution (as a biological term) was first applied. But not 

 only did they compare the germ to a complex bud hiding 

 the already formed organs within its hull, they maintained 

 that it included also the next generation and the next and 

 the next. Some said that the ovum was most important, 

 that it required only the sperm's awakening touch and it 

 began to unfold ; others said that the animalcules or 

 spermatozoa produced by male animals were most im- 

 portant, that they only required to be nourished by the 

 ova. The two schools nicknamed one another " ovists " 

 and " animalculists." The preformation-theories were false, 

 as Harvey in the middle of the seventeenth century discerned, 

 and as Wolff a century later proved, because germs are 

 demonstrably simple, and because embryos grow gradually 

 part by part. But in a later chapter we shall see that the 

 theories were also strangely true. 



4. The Egg-cell or Ovum produced by a female animal, 

 or at least by a female organ (ovary), exhibits the usual 

 characteristics of a cell. It often begins like an Amoeba, 

 and may absorb adjacent cells ; in most cases it becomes 

 surrounded by an envelope or by several sheaths ; in 

 many cases it is richly laden with yolk derived from various 

 sources. In the egg of a fowl, the most important part 

 (out of which the embryo is made) is a small area of trans- 

 parent living matter which lies on the top of the yellow 

 yolk and has a nucleus for its centre ; round about 

 there is a coating of white-of-egg ; this is surrounded by 

 a double membrane which forms an air-chamber at the 

 broad end of the t.^'g ; outermost is the porous shell of 

 lime. 



While there must be a general relation between the size 

 of the bird and that of the e.%^, there are many inconsisten- 

 cies, as you will soon discover if you compare the eggs 

 of several birds of the same size. It is said that the eggs 

 of birds which are rapidly hatched and soon leave the nest 

 tend to be large, and that there is some relation between the 

 size of eggs and the number which the bird has to cover. 

 It seems probable, however, from what one notices in the 



