236 THE POPULAR SCIENCE MONTHLY 



of structures or characters de novo. From beginning to end develop- 

 ment in a series of morphological and physiological changes, but not of 

 new formations or creations. It is only the incompleteness of our 

 knowledge of development which allows us to say that the eye or ear or 

 brain begin to form in this or that stage. They become visible at cer- 

 tain stages, but their real beginnings are indefinitely remote. 



II. Correlations Between Germinal and Somatic Organization 



All the world knows that the organization of the germ is not the 

 same as that of the developed animal which comes from it, and yet the 

 specificity of the germ indicates that there must be some correlation 

 between the germinal and the developed organization — in short, there is 

 not identity of organization, but correlation of organization between the 

 germ and the adult. What correlations are known to exist between 

 the oosperm and the developed animal ? 



1. Nuclear Correlations 



Many biologists maintain that the nucleus and more particularly 

 the chromosomes are the exclusive seat of the " inheritance material " 

 and that all the "determiners" of adult characters are located in them. 



There are certain general and a priori reasons for assuming that the 

 chromosomes are important factors in heredity and differentiation; 

 (1) they come in approximately equal numbers from the father and 

 the mother, (2) one half of each of the maternal and paternal chromo- 

 somes is distributed to each cell of the developing organism, (3) in the 

 formation of the egg and sperm cells the normal number of chromosomes 

 is reduced by one half, and (4) in fertilization the normal number is 

 restored by the union of the chromosomes of the egg and sperm. It is 

 a remarkable fact that the determiners or factors of certain inherited 

 characters come in equal numbers from both parents and that in spite 

 of their ultimate association in an individual they may be separated or 

 "segregated" in the formation of that individual's germ cells. Such 

 inheritance is known as Mendelian and will be treated at length in the 

 next lecture, but it may be said here that the association, distribution 

 and segregation of Mendelian factors and of maternal and paternal 

 chromosomes is exactly parallel. This is strong evidence that these 

 factors are associated with the chromosomes. 



There are also certain special reasons for considering that the 

 chromosomes are important factors in heredity and development. (5) 

 Boveri has studied the abnormal distribution of chromosomes to dif- 

 ferent cleavage cells in doubly fertilized sea-urchin eggs, and has found 

 evidence that the hereditary value of different chromosomes is different. 



(6) McClung, Stevens and Wilson have discovered that the determi- 

 nation of sex is associated with the presence or absence of a particular 

 chromosome, the X-chromosome, in the spermatozoon which fertilizes 



