FROM SIMPLE CELL TO COMPLEX ANIMAL 45 



final scene in the mixing of male and female qualities which 

 began when the egg and sperm united in fertilization. The 

 protoplast of ovum and sperm unite at fertilization; but the 

 final union of the chromosomes of the ovum and sperm is de- 

 layed until near the time of the formation of the next generation 

 of sperm and ova. It is possible that these bivalent chromo- 

 somes may exchange material before their final separation. 



The reduction of the chromosomes to one-half their typical 

 number is clearly a device which maintains the standard number 

 when the ovum and sperm unite. If they were not reduced the 

 number of the chromosomes, and the accompanying hereditary 

 effects, would be doubled at every fertilization. 



6i. Summary. 



1. All the higher animals begin life as a single cell and reach 

 their adtdt condition by a continuous series of divisions. By 

 the growth and specialization of the cells arising from these 

 divisions the great complexity of the adtilt body is produced. 



2. This initial cell — the fertilized ovum — represents the 

 fusion of two independent and unlike cells: the ovum (female) 

 and the spermatozoon (male). 



3. Before the union (fertilization) occurs, the ovum reduces 

 , its nuclear material, by two successive divisions, to one-fourth 



its original amount and the chromosomes to one-half their 

 original number, without a corresponding reduction of the cyto- 

 plasm. The spermatozoon in its development undergoes a 

 similar reduction of chromosomes and a great reduction of pro- 

 toplasm. When fertilization occurs the full number of chromo- 

 somes is restored to the new organism. This reduction and 

 restoration of chromatin is believed to play a large part in 

 inheritance. 



4. After the union of the male and female cells the fertilized 

 ovum divides rapidly (segmentation or cleavage) forming a mass 

 of cohering cells. The nature of these cells and of the mass 

 depends much on the amount of yolk in the ovum and on its 

 distribution. 



5. By processes which differ in different animals according 

 to the nature of the segmentation, the cells become arranged 



