DEVELOPMENT. 25 



evidence that these factors are lodged in the chro- 

 matin of the cell nucleus. 



Environment is the other great factor that brings 

 about a structural modification. It is between 

 environment on the one hand, and heredity on the 

 other, that a specialization and differentiation of 

 cells, tissues, and organs is produced. 



A cell is a spacially limited mass of protoplasm 

 which, under certain conditions, will assimilate, grow, 

 and reproduce itself. A tissue is a complex of simi- 

 larly differentiated cells and their derivatives. An 

 organ is a complex of tissues, forming a body with a 

 definite internal structure and 

 external form. 



Fig. 2. Portions of the ova of Asterias glacialis, showing the ap- 

 proach and fusion of the spermatozoon with the ovum (Hertwig): a, 

 fertilizing male element; b, elevation of protoplasm of egg; b', b", stages 

 of fusion of the head of the spermatozoon with the ovum. 



Ovulation and Maturation. The ova develop in 

 the ovaries and are differentiated very early during 

 embryonic life. The estimated number of ova in each 

 ovary is 35,000. It is a remarkable phenomenon that 

 for many years these units show no attempt at devel- 

 opment or cell division. At the age of puberty one or 

 more of these cells pass periodically from the human 

 ovaries, approximately every twenty-eight days in the 



