306 THE STUDY OF ANIMAL LIFE CHAP. 



X 



off as a free-swimming ephyra which eventually becomes 

 a sexual jellyfish (see fig. 65). 



Similar but sometimes more complicated alternations 

 occur in some worm-types (some flukes, threadworms, 

 etc.), and as high up in the series as Tunicates ; while 

 among plants analogous alternations are very common, 

 e.g. in the life-cycles of fern and moss. 



4. Historical. In the seventeenth and eighteenth cen- 

 turies, naturalists had a short and easy method of dealing 

 with embryology. They maintained that within the seed 

 of a plant, within the egg of a bird, the future organism 

 was already present in miniature. Every germ contained 

 a miniature model of the adult, which in development was 

 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 awaken- 

 ing touch and it began to unfold ; others said that the 

 animalcules or spermatozoa produced by male animals 

 were most important, that they only required to be 

 nourished by the ova. The two schools nicknamed one 

 another " ovists ' and " animalculists." The preforma- 

 tion-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. 



5. 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 transparent living matter which 



