PROTOZOA TO WORMS 



43 



correspondence from the fact already stated that the 

 embryonic development of the individual is a brief re- 

 capitulation of the ancestral development of the spe- 

 cies or larger group. The egg of the lowest vertebrate, 

 amphioxus, shows these changes in a simple and ap- 

 parently primitive form. 



The fertilized egg of any animal consists of a single 

 cell, a little mass of protoplasm containing a nucleus 

 and surrounded by a struct- 

 ureless membrane. The egg 

 is globular. The nucleus 

 undergoes certain very pe- 

 culiar, still but little under- 

 stood, changes and divides 

 into two. The protoplasm 

 also soon divides into two 

 masses clustering each 

 around its own nucleus. 



The plane of division will 3. IMMATURE EGG-SHELL FROM OVA- 



-, i -I -i , i RY OF ECHINODERM. HATSCHEK, 



be marked around the out- FROM HEKTWIG . 



side by a circular furrow, 



but the cells will still remain united by a large part of 



the membrane which bounds their adjacent, newly 



formed, internal faces. 



Let us suppose that the egg lay so that the first 

 plane of division was vertical and extending north and 

 south. Each cell or half of the egg will divide into 

 two precisely as before. The new plane of division 

 will be vertical, but extending east and west. Each 

 plane passes through the centre of the egg, and the 

 four cells are of the same form and size, like much- 

 rounded quarters of an orange. The third plane will 

 lie horizontal or equatorial, and will divide each of 



