336 ANIMAL DEVELOPMENT 



history, i.e. the egg-cell, stands for the original one-celled ancestors 

 of the many-celled animals. This gradual building up of the 

 complex body of an animal is one of the most astonishing facts 

 with which zoology has to deal, and can only be explained on an 

 evolutionary basis. The early students of development held views 

 which were totally at variance with the facts as we now know 

 them. They believed, for instance, that the youngest discernible 

 stage in the growth of the chicken within the o.'g'g differed from 

 the adult bird only as regards size. This was the doctrine of 

 Preformation, according to which development merely consisted 

 in the enlargement of parts already present, by a process of 

 " unfolding " or " evolution ". The last expression was used, it 



will be noticed, in 

 quite a different 

 sense from the 

 modern one. The 

 rival doctrine of 

 Epigenesis, accord- 

 Fig. S59.— Formation of Polar Bodies, diagrammatic l'^§ tO WnlCh the 



In A and B is shown successive formation of first and second polar bodies L'vJUy lo ^IdClUdliy 

 (i and 2). c, Egg-cell ready for fertilization: i and 2, first and second polar VinJlf iiri in fVlf^ \J,T^\T 

 bodies ; }i, nucleus. i^ J 



already indicated, 

 owes its first inception to the gifted investigator, Caspar Friedrich 

 Wolff, who flourished in the latter part of the eighteenth century. 

 In the realm of botany the errors of the Preformation theory 

 linger even yet. The present writer has had the doubtful 

 privilege of hearing eloquent discourses embellished by such 

 statements as: "The tiny acorn contains in miniature every root, 

 twig, and leaf of the mighty oak, of which it is the parent ". 



Polar Bodies (fig. 859). — Before an egg-cell is mature and 

 ready to be fertilized it twice undergoes unequal division, of which 

 the smaller products are known as "polar-bodies". They take no 

 part in the development, but sooner or later disintegrate. As a 

 result of their extrusion the egg-cell gets rid of three-quarters of 

 its nuclear substance. The maturation of sperms is associated 

 with a similar process. Why polar bodies should be formed at all 

 is by no means clear, though they have been the object of a large 

 amount of investigation and speculation. It is quite likely that, 

 for one thing, the egg-cell is, so to speak, weakened, and thereby 

 prevented from developing into an adult animal without fertiliza- 



