Fig. 46. A series of cells in process of cell division. The chromosomes are the black threads and rods. (After Dahlgren.)
The story is taken up at this point by a new set of students who have worked in an entirely different field. Certain observations and experiments that we have not time to consider
now, led a number of biologists to conclude that the chromosomes are the bearers of the hereditary units. If so, there should be many such units carried by each chromosome, for the number of chromosomes is limited while the number of independently inherited characters is large. In Drosophila it has been demonstrated not only that there are exactly as many groups of characters that are inherited together as there are pairs of chromosomes, but even that it is possible to locate one of these groups in a particular chromosome and to state the relative position there of the factors for the characters. If the validity of this evidence is accepted, the study of the cell leads us finally in a mechanical, but not in a chemical sense, to the ultimate units about which the whole process of the transmission of the hereditary factors centers.
But before plunging into this somewhat technical matter (that is difficult only because it is unfamiliar), certain facts which are familiar for the most part should be recalled, because on these turns the whole of the subsequent story.
Fig. 47. An egg, and the division of the egg—the so-called process of cleavage. (After Selenka.)
The thousands of cells that make up the cell-state that we call an animal or plant come from the fertilized egg. An hour or two after fertilization the egg divides into two cells (fig. 47). Then each half divides again. Each
quarter next divides. The process continues until a large number of cells is formed and out of these organs mould themselves.
