REPRODUCTION 95 



the egg cell. The two unite in the tube and the egg 

 gains the power of division and differentiation. In in- 

 sects, birds, reptiles, after the process of fertilization oc- 

 curs, a hard outer wall forms around the eggs. They 

 can then be exposed to the air without injury. The 

 process of differentiation and growth continues within 

 the shell until the young animal can take care of itself, 

 when it is hatched. In higher animals, where a hard 

 shell does not form, the egg can not be exposed to the air 

 until it is so protected that no harm will result. There- 

 fore, the length of time that the fertilized egg remains 

 in the tube and the amount of development which takes 

 place there depend upon the degree of development which 

 is necessary for the protection of the new individual. 

 In the case of human beings the new individual must 

 be completely formed before it leaves the tube. 



Sexual Reproduction of Plants. — In plants the proc- 

 ess is essentially the same. The ultimate end and aim 

 of all organisms seems to be the reproduction of their 

 own kind ; the flower is not made to please our eyes but 

 to aid in the production of a new plant. The brightly 

 colored floral envelopes usually present surround the 

 more important stamens and pistils (Fig. 58). These 

 may, or may not, appear in the same flower. The pistil 

 is a tube (the style), with a basal enlargement (the 

 ovary) and a flaring end (the stigma). In the ovary, as 

 the name indicates, the ovules, or egg cells, are formed. 

 Each of these may be compared to a female sea-urchin, 

 for it forms within itself a little egg cell which must 

 be fertilized if it is to develop into a new individual. 



