FACTS AND FACTORS OF DEVELOPMENT 23 



instincts, habits, and intelligence of the ma- 

 ture animal. 



Is not this miracle of development more 

 wonderful than any possible miracle of cre- 

 ation? And yet as one watches this marvellous 

 process by which the fertilized egg grows into 

 the embryo, and this into the adult, each step 

 appears relatively simple, each perceptible 

 change is minute; but the changes are in- 

 numerable and unceasing and in the end they 

 accomplish this miracle of transforming the 

 fertilized egg cell into the fish, or frog, or 

 man — a thing which would be incredible were 

 it not for the fact that it has been seen by 

 hundreds of observers and can be verified at 

 any time by those who will take the trouble to 

 study the process for themselves. 



3. Cleavage. — After its entrance into the 

 egg the sperm nucleus moves toward the egg 

 nucleus until the two meet when they divide 



B and C, Later stages showing the separation of the daughter 

 chromosomes from one another and their movement toward the 

 two poles of the spindle. D and E, Still later stages showing 

 the swelling of the chromosomes and their fusion to form nuc- 

 lear vesicles. F, Complete division of egg into two cells, each 

 containing one daughter nucleus and centrosome. (After 

 Boveri.) 



