SYMBIOLOGY THE BIOLOGICAL RELATIONSHIPS OF ORGANISMS 127 



medium with a given species of paramecium and keep the food supply 

 constant and uniform, we find that sooner or later the organisms gradually 

 divide or septate less actively until finally all septa tion ceases and the 

 medium becomes freed of living paramecia. In other words, the organ- 

 ism has completed its cycle of somatic existence, which may have lasted 

 hundreds and thousands of generations. By means of certain stimuli, as 

 strychnine, alcohol, etc., the cycle may indeed be prolonged by many 

 additional generations but even these stimuli will not make it possible to 

 maintain the cycle indefinitely. In the case of bacteria, biological cycles 

 have been maintained for many years, apparently without appreciable 

 diminution of the septating powers of the cells, but bacteriologists declare 

 that sooner or later a strain of bacteria will deteriorate and take on ir- 

 reparable changes. In a similar manner the somatic cells of a many- 

 celled animal complete the biological cycle and somatic death follows. 

 In the one case the individual cells are apparently all alike and live apart, 

 and in the second case the individual cells differ morphologically as well as 

 physiologically and are united by growth and have become specialized 

 into tissues and organs. Just as the continued existence of the single- 

 celled race is made possible through the gametic relationships of certain 

 cells just so the life of the somatic cells of higher plants and animals is 

 made continuous through the reproductive cells. 



Within recent years many researches have been made in cytology and 

 embryology. Of these the most interesting were the discovery of the sex 

 chromosomes and the relationship of these bodies in the male and 

 female gametes, by Boveri, Henking, McClung, Wilson and others. The 

 chemism of the egg cell received the attention of Baltzer, Herbst, Loeb and 

 many others. It was found that the egg cell of the sea urchin, of the frog 

 and of other animals, could be induced to segment without the presence of 

 the male cell (Herbst, Loeb). The most spectacular discovery was that 

 of the sex determinant in the chromosomes of the male or sperm cell. It 

 was found that the germ cells of higher plants and animals differ from the 

 somatic cells in the reduction of the chromosome bands (24 in the somatic 

 cells as against 1 2 in the germatic cells, as a rule) . The 1 2 chromosomes 

 of the female germ cell fuse making 6 larger chromosomes. In the male 

 germ cell a most remarkable irregularity in the union of the chromosomes 

 was observed, upon which irregularity the determination of sex depends. 

 Some of the sperm cells contain 1 2 chromosomes and an extra larger chro- 

 mosome, which is the sex determining chromosome. There are therefore 

 two kinds of male reproductive cells, one with six fused chromosomes and 

 the other with seven chromosomes (six fused and one single) . The latter 

 are the male producing sperm cells. The male and female producing 

 sperm cells are supposed to be present in about equal numbers. Which 



