Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER IX THE CHROMOSOMES AND MITOCHONDRIA OF GERM CELLS No account of the germ-cell cycle in animals can be considered complete without at least a brief reference to the history of the chromosomes and mitochondria of germ cells. The chromosomes have for many years been recognized as the most important visible bodies in the cell, and their behavior during the germ- cell cycle has convinced most zoologists that they may also be regarded as the bearers of hereditary factors. The mitochondria, on the other hand, are cellular constituents which have only comparatively recently come into prominence in cytological literature, and ideas concerning their nature and functions are still in a very chaotic condition. The Chromosome Cycle In Animals A few general statements regarding the behavior of the chromosomes during cell division, maturation, and fertilization are contained in Chapters I and II. We may recognize a rather definite chromosome cycle as a part of the germ-cell cycle, and it is to certain events in this chromosome cycle that our attention will be directed in the following paragraphs. It is best to begin our discussion, as in the general reviewof the germ-cell cycle (Chapter II), with the par- thenogenetic or fertilized egg after the maturation processes have been completed, and to exclude all references to the accessory chromosome until later. It may be pointed out first that the number of chromosomes in the cells of any individual of a species is, with few exceptions, constant. Thus the thread worm of the horse, Ascaris megalocephala var. univalens, has two; A. megalocephala var. bivalens, four; the nematod, Coronilla, eight; the mole cricket, Gryllotalpa vulgaris, twelve; the bug, Pentatoma, fourteen; the rat, sixteen; the sea urchin, Echinus, eighteen; the ... --This text refers to an alternate Paperback edition.