30 Heredity and Environment 



posed "maternal impressions" of the physical, mental, or emo- 

 tional conditions of the mother upon the unborn child have no 

 existence in fact, except in so far as the quality of the mother's 

 blood may be changed and may affect the child. At no time, 

 whether before or after birth, is the mother more than nurse 

 to the child. Hereditary influences are transmitted only through 

 the egg cell and the sperm cell and these influences are not affected 

 by intra-uterine development. The principles of heredity and 

 development are the same in oviparous and in viviparous animals 

 in fishes, frogs, birds and men. 



Summary. This is a very brief and incomplete statement of 

 some of the important stages or phases of the development of the 

 body of man or of any other vertebrate. In all cases development 

 begins with the fertilized egg which contains none of the struc- 

 tures of the developed animal, though it may exhibit the polarity 

 and symmetry of the adult and may also contain specific kinds of 

 protoplasm which will give rise to specific tissues or organs of the 

 adult. From this egg cell arise by division many cells which dif- 

 fer from one another more and more as development proceeds, 

 until finally the adult animal results. A specific type of develop- 

 ment is due to a specific organization of the germ cells with which 

 development begins, but the earlier differentiations of the egg are 

 relatively few and simple as compared with the bewildering com- 

 plexities of the adult, and the best way of understanding adult 

 structures is to trace them back in development to their simpler 

 beginnings and to study them in the process of becoming. 



7. Development of Functions. The development of functions 

 goes hand in hand with the development of structures; indeed 

 function and structure are merely different aspects of one and 

 the same thing, namely organization. All the general functions 

 of living things are present in the germ cells, viz., (i) Construc- 

 tive and destructive metabolism, (2) Reproduction, as shown in 

 the division of cells and cell constituents, (3) Irritability, or the 

 capacity of receiving and responding to stimuli. All these gen- 

 eral functions of living things are manifested by germ cells, but 



