Other chromosome mutations also occur in rare instances, and each usually produces a detectable change in the individual. A whole extra chromosome may be received by a germ cell owing to the failure of a particular pair to divide. This occurrence naturally leaves one germ cell with n - 1 chromosomes. In such cases the deficient germ cell usually does not function. The redundant germ cell may occasionally mate with a normal germ cell, however, with the result that an individual is produced having 2n + 1 chromosomes. Other chromosome mutations have also been traced, and their study has yielded significant results. Pieces of chromosomes are lost; unequal crossing-over occurs; and there are duplications and translocations of chromosome parts. Following such instances in pedigree cultures by means of known genes has established the Chromosome Theory of Heredity beyond all shadow of doubt, and has thrown new light on the way in which evolution has taken place.

This brief glimpse of one of the main phases of genetic work gives fair support1 for the statement that the me 1 Detailed proof of the genetic principles mentioned (and many others unmen-tioned), together with the consequences which flow from them, can be found in numerous elementary textbooks."The Principles of Genetics," by E. W. Sinnott and L. C. Dunn (McGraw-Hill Book Company, Inc., New York, 1925), and"Genetics in Relation to Agriculture," by E. B. Babcock and R. E. Clausen (McGraw-Hill Book Company, Inc., New York, 2d ed., 1927), are recommended. For information on the cellular basis of genetics, the reader should refer to "he Cell in Development and Heredity," by E. B. Wilson (The Macmillan Company, New York, 3d ed., 1925). For the effect of modern genetic doctrines on the evolution concept, he should see "A Critique of the Theory of Evolution,"by T.

chanical problem of heredity has been solved as definitely as the mechanical problem of molecular structure. Let us recapitulate. Every organism, whether man or beast, whether flowering plant or fern, starts life as a combination of more or less independently inherited genes. The genes are the units of heredity, the physical connection between one generation and its successor, and are the basis of all developmental changes. They are self-perpetuating bodies which grow and divide throughout long periods of racial history without exhibiting discernible variation in the functions they perform; yet in rare cases any given gene may take on a new constitution and thus, if a functional germ cell results, provide new raw material for the sieve of natural selection. The gene pattern of every individual is definite and distinctive for that individual, and is ordinarily made up of two complete sets of genes, one contributed by the egg and one by the sperm. Each mature egg or sperm possesses one complete set of genes, chosen from these two sets by means of a regularly constituted method of gene distribution. Normally, the essential features of this distribution are as follows: Any homologous pair of chromosomes may exchange one or more genes, provided the exchange is equable and is carried out in obedience to regulations. The entire packet of genes of each respective pair of chromosomes is then passed to the two daughter cells which become the mature reproductive cells - through a second equatorial division having no known genetic significance - by the free assortment of one member of each pair.

The emphasis which has been placed upon gene patterns should not be construed as meaning that no other factors have determinative value in the final result that is called H. Morgan (Princeton University Press, Princeton, 1916), or "The Genetical Theory of Natural Selection," by R. A. Fisher (The Clarendon Press, Oxford, 1930). the organism. Such an idea would be incorrect. The fertilized egg has a gene pattern, it is true, and the type of gene pattern which it possesses has a governing influence on the characteristics of the adult; but the fertilized egg also contains cytoplasm, and cytoplasmic activities are by no means to be neglected. Further, a fertilized egg must meet hospitable conditions in order to develop; and these conditions may be varied. And finally, the behavior of the individual which results from the interaction of gene pattern, cytoplasm, and environment, is also conditioned by experience.

Cytoplasm is the term used for the mass of protoplasm in which the nucleus containing the chromosomes is embedded. Its precise function is not yet wholly clear. Perhaps we should not go far wrong if we considered it to be simply one of the internal factors of environment. It appears to contain no essential substances which, like the chromosomes, must be transferred to the daughter cells at each cell division in order to have the machine function effectively. On the contrary, the cytoplasm seems to be built up anew each time a cell divides, as if it were being manufactured under the direction of the nucleus. Nevertheless, it plays an extraordinary part in early development. In certain animals it can be shown that the cytoplasm of the unfertilized egg gradually arranges itself into three strata, and that these strata constitute the groundwork of the new individual. For example, in the sea urchin one layer becomes the body covering, one the inner part of the alimentary tract, and one the skeleton; and if any layer is disturbed, there is a corresponding disturbance in these particular parts. Now this cytoplasmic behavior may be directed by the chromosomes, and there is some evidence that this is so; but at all events, whatever structural differentiation takes place in the unfertilized egg occurs under the sole influence of the genes of the mother. The general ground plan of the body, perhaps the common characteristics of the order or the family, is due to the mother alone. Another plausible conclusion can also be drawn from these facts. Since if one of the cytoplasmic layers of the sea urchin is removed by a tiny pipette, the part of the body to which it should give rise does not develop, it appears to follow that the nucleus can perform its governmental function but once in a given cell generation.