II 



NORMAL DEVELOPMENT 



For the worker who aims to follow the normal development 

 of a marine egg in the laboratory, the primary consideration is 

 the assurance that the processes which he observes are the same 

 as those that occur in nature. Descriptive embryology built 

 upon laboratory observations stands only if one can assume that 

 the stages observed represent faithful reproductions of those 

 occurring in the state of nature. Thus, for the descriptive 

 embryologist the normality of the object and the knowledge of 

 this quality are most important. The acquisition of this knowl- 

 edge is a task of no mean proportions because in no small degree 

 is the work in the laboratory on descriptive embryology an 

 experimental one since, in the best circumstances even, marine 

 eggs are under not natural conditions. 



The task of the experimental embryologist is still more 

 onerous. He aims to make his experiments in such wise that 

 they are capable of being repeated not only by himself but by 

 others, reducing to the lowest limit the personal factor which 

 enters so largely in biological investigations. He is, or should 

 be, therefore, vigilant in repeating an experiment to reproduce 

 all the circumstances attending it: working with the same vol- 

 umes of eggs and of sperm in the same volume of sea-water of 

 the same quality and at the same temperature. Variations in 

 these factors being themselves experimental must be rigorously 

 controlled. Also, for the experimental embryologist are the 

 normality of the egg and the recognition of this quality basic 

 prerequisites. 



The basis and the control of any experiment is the perfectly 

 normal egg; the worker must know therefore what is a good 

 egg. The best source for this knowledge lies in the most thor- 

 ough acquaintance of the normal egg in its normal surroundings. 

 Whenever possible the normal development of the egg in nature 

 should be followed. 



7 



