Doncaster: Determination of Sex 



271 



and differing from ordinary protoplasm 

 in containing a quantity of a substance 

 called chromatin, so called because it 

 takes up stains (Greek, chroma) more 

 readily than other parts of the cell. 

 In its ordinary condition the chromatin 

 is scattered in fine granules on proto- 

 plasmic threads enclosed in the nuclear 

 membrane. We know that the nucleus 

 is of fundamental importance to the 

 life of the cell; metaphorically we may 

 say that the nucleus is to the cell what 

 the brain is to the body. When a 

 spermatozoon meets an egg cell, it 

 forces its way into it, until its head is 

 embedded in the egg 'protoplasm; the 

 tail is then dropped off, and the head, 

 which consists almost entirely of a very 

 concentrated nucleus, swells up until 

 it reaches the size of the nucleus of the 

 egg-cell. The two nuclei then slowly 

 approach each other until they meet, 

 when they fuse into a single zygote- 

 nucleus, which immediately begins to 

 divide in such a way that equal parts 

 of both parental nuclei are contained 

 in each half. The whole cell then 

 divides in two, and the process is re- 

 peated until an embryo containing 

 thousands of cells is produced. In 

 every division the nucleus is divided in 

 such a way that both paternal and 

 maternal portions are accurately halved ; 

 from this it results that every cell of 

 the offspring contains equal parts of 

 the paternal and maternal nuclear 

 substance (chromatin)." It is by the 

 study of the details of this process of 

 division that more light has been thrown 

 on the problem of sex-determination, 

 perhaps, than by any other method. 



MIXTURE OF INHERITANCE 



"In the process of fertihzation we 

 get almost our only definite indication 

 of the ultimate nature and function of 

 sex. We have seen that every part of 

 every individual includes equal portions 

 of nuclear matter descended from one 

 and the other parent. The mechanism 

 for producing this equal division is 

 extremely beautiful and complex, and 

 it is impossible to believe that it is not 

 of fundamental importance. There is 

 evidence that the nucleus, and particu- 

 larly its chromatin, is especially con- 



cerned in the transmission of inherited 

 characters, and the mechanism of nu- 

 clear division insures that, of this 

 'material basis of heredity,' the portion 

 derived from each parent shall be equally 

 distributed to every part of the body. 

 One of the chief effects, then, of sexual 

 reproduction, and perhaps its most 

 important function, is the equal min- 

 gling in every individual of sets of inher- 

 ited characters derived from two 

 parents." 



But such a result, whatever its advan- 

 tages to the organism and the species 

 may be, might also occur in the absence 

 of two distinct sexes. "If all individ- 

 uals produced similar germ-cells, and 

 if these united with one another at 

 random, we should still get fertilization 

 and a similar recombination of char- 

 acters, although there would be no 

 division into males and females." Such 

 a condition is, in fact, found in some of 

 the simplest one-celled animals and 

 plants, such as Protozoa, where two 

 similar individuals fuse to form one. 

 "In the higher, multicellular animals, 

 such union of similar cells is never 

 found, and even in the Protozoa it is 

 usual that the two cells which conjugate 

 (i. e., unite in fertilization) should be 

 dissimilar. One cause for this is not 

 difficult to discover. The zygote formed 

 by conjugation of two germ-cells has 

 to grow up directly into a new indi- 

 vidual, and if the function of fertilization 

 referred to above is to be fulfilled, the 

 germ-cells must come from different 

 parents. If both cells were alike, it 

 would be much more difficult for them 

 to meet each other than if one is rela- 

 tively large and stationary, the other 

 small and active. Further, it is a 

 great advantage to the embryo that it 

 should be provided with some source of 

 nourishment in its early stages. This 

 is done in many cases by the storing 

 up of food-material (yolk) in the egg, 

 and it would clearly be impossible for 

 two such yolk-laden eggs to seek each 

 other out and unite. In a number of 

 animals belonging to various groups, 

 and also in the flowering plants, the 

 embryo is supplied with nourishment 

 direct from the mother, and this again 

 would be impossible if both germ-cells 



