ONTOGENY OF METAZOA 153 



2. Fertilization, or the Formation of the Zygote (C). 

 After maturation, the sperm cells move to the eggs, if 

 conditions permit, and fuse with them. Only one sperm 

 cell unites with each egg, though many may try. The 

 sperm cell is always much smaller than the egg. Sometimes 

 its bulk is only one five-hundred thousandth that of 'the 

 ovum with which it unites. It usually pierces the cyto- 

 plasm of the egg for a short distance. Before fertilization 

 takes place both the germ cells have half the somatic 

 number of chromosomes. When the sperm enters the egg, 

 it brings chromatin and chromosomes equal to those in 

 the egg, and the somatic condition is thus restored in the 

 zygote. 



3. Cleavage. Soon after the sperm cell has entered the 

 egg chromosomes are formed in both egg and sperm 

 nuclei, a spindle is originated from the centrosomes of the 

 sperm nucleus, and the chromosomes take position around 

 its equator (C). These chromosomes are half from the 

 sperm cell and half from the egg. The usual steps in mito- 

 sis (page 40) are now completed arid the " fertilized egg" 

 or zygote, thus divides into two cells. This division is 

 known as the first cleavage. There follow two-, four-, 

 eight-, sixteen-, thirty-two-celled " cleavage stages" (D); 

 and others involving even larger numbers of cells. After a 

 time a little ball of cells, the morula, is formed and this 

 later becomes hollow by having a water-filled " cleavage 

 cavity" (x) form in the middle. Such a ball of cells with 

 a central cavity is called a blastula (E). It soon changes 

 to a gastrula by the turning in of one side. 



4. Gastrulation; the Formation of the Germ Layers. 

 When the side of the hollow spheroidal blastula is turned in, 

 the cleavage cavity (x) is encroached upon and is finally 

 completely obliterated because the turned in portion 

 comes in contact with the epithelium on the other side of 

 the original sphere (F, G). The result is a concave hemi- 

 sphere made up of two layers. The epithelium covering 

 the outside is the ectoderm (ec); the cavity within is the 



