310 THE STUDY OF ANIMAL LIFE CHAP. 



the same " complex factorial background," and regional 

 peculiarities of different parts of the developing embryo 

 determine which shall find expression and which shall be 

 latent. There is no sorting out to appropriate cells, but 

 each receives the complete hereditary organisation. To 

 return to our metaphor, each plot receives the whole 

 collection of seeds, but environmental conditions of soil, 

 temperature, position, and the like determine that only 

 certain seeds will develop in each plot. In any case the 

 student must realise that the most familiar of processes 

 -differentiation remains very mysterious. 



3. Some Generalisations. (a) The "Ovum-Theory." 

 To realise that almost every organism from sponge to 

 man begins its life as a fertilised egg-cell, and is built up 

 by the division and arrangement, layering and folding of 

 cells, should not lessen, but should greatly enhance, the 

 wonder with which we look upon life. If the end of this 

 constantly repeated process of development be something 

 to marvel at, the same is equally true of its beginning. 



(b) The Gastrcea Theory. From the frequent, though 

 not universal occurrence of the two-layered gastrula stage 

 in the development of animals, Haeckel concluded that 

 the first stable form of many-celled animals must have 

 been something very like a gastrula. He called this hypo- 

 thetical ancestor of all many-celled animals a Gastrcea, 

 and his inference has found favour with many naturalists. 

 Some of the simplest sponges, polyps, and " worms ' 

 arc hardly above the gastrula level. 



(c) Recapitulation. Von Baer, one of the pioneer 

 cmbryologists in the first half of the nineteenth century, 

 discerned that the individual development is from the 

 general to the special. He recognised that even one of 

 the higher animals, let us say a rabbit, first puts on the 

 features of a primitive Vertebrate, that it subsequently 

 shows the character of an embryo fish, then of an em- 

 bryonic reptile, then of an embryonic mammal, then of 

 a young rodent, finally of a young rabbit. He confessed 

 his inability to tell whether a number of very young 

 embryos, freed from their surroundings, were those of 

 reptiles, birds, or mammals. In stating Von Baer's 



