Page:Popular Science Monthly Volume 85.djvu/240

236 of structures or characters de novo. From beginning to end development 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 certain stages, but their real beginnings are indefinitely remote.

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?

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 chromosomes 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 different 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 determination of sex is associated with the presence or absence of a particular chromosome, the X-chromosome, in the spermatozoon which fertilizes