l82 UNIVERSITY OF MISSOURI BULLETIN 



As a result of the older descriptive embryology, we have 

 followed stage by stage the development of many representa- 

 tives from all the great groups and have no longer crude 

 misconceptions of the links by which one generation of adult 

 forms is connected with another, and yet, the wonder of 

 development has increased the more. Despite our wealth of 

 observation, we must confess that we know very little of how 

 adult living things come into being, save for the mere visible 

 changes by which the egg becomes the adult. These we know 

 fairly well from descriptive work, but they are not satisfying. 

 We wish to know why one stage follows another and why 

 this or that event occurs at a certain time, and to ascertain 

 facts of this nature, we must have recourse to the experimental 

 method. This may be illustrated by some particular cases. 



The phenomenon of fertilization, by which is meant the 

 union of the egg and spermatozoon, is observed to be the 

 beginning of individual life in most animals. If isolated, the 

 spermatozoa will die and so will the eggs. If brought to- 

 gether, in such a way that the normal union of a single sperm 

 with a single egg may take place, the development follows 

 and eventually the adult form is produced. Why is it that 

 the entrance of the spermatozoon causes this profound dif- 

 ference in the fate of any egg? This is one of the problems 

 which experimental embryology is seeking to answer. 



Even under normal conditions, it is not necessary for 

 the eggs of all animals to become fertilized, for we have long 

 known of the phenomenon of parthenogenesis by which eggs 

 may develop without the entrance of a spermatozoon. In 

 some few cases, the failure to discover males in any genera- 

 tion indicates that no male sex exists, though in these it is 

 possible that males appear so rarely as to have been over- 

 looked. In most cases of parthenogenesis, however, we find 

 males appearing in certain generations and here the fertiliza- 

 tion of the eggs is necessary for their development. Accord- 

 ingly, in parthenogenesis as it normally occurs, we see that 



