The first one to recognize the existence of two kinds of spermatozoa was Henking, who stated that in _Pyrrhocoris_ (a Hemipteran) one-half of the spermatozoa of each male possessed a nucleolus, while in the other half it was lacking. Montgomery afterward showed that Henking’s nucleolus was an accessory chromosome. McClung was the first to recognize the importance of this fact for the problem of sex determination. He observed an accessory chromosome in one-half of the spermatozoa of two forms of Orthoptera, _Brachystola_ and _Hippiscus_, and reached the following conclusion: A most significant fact, and one upon which almost all investigators are united in opinion, is that the element is apportioned to but one-half of the spermatozoa. Assuming it to be true that the chromatin is the important part of the cell in the matter of heredity, then it follows that we have two kinds of spermatozoa that differ from each other in a vital matter. We expect, therefore, to find in the offspring two sorts of individuals in approximately equal numbers, under normal conditions, that exhibit marked differences in structure. A careful consideration will suggest that nothing but sexual characters thus divides the members of a species into two well-defined groups, and we are logically forced to the conclusion that the peculiar chromosome has some bearing upon the arrangement. N. M. Stevens and E. B. Wilson[183] have not only proved the correctness of this idea for a number of animals but have laid the foundation of our present knowledge of the subject. Wilson showed that in those cases where there are two types of spermatozoa, one with and one without an accessory or as it is now called an X chromosome, all the cells of the female have one chromosome more than the cells of the male. From this he concludes correctly that in such species a female is produced when the egg is fertilized by a spermatozoön containing an X chromosome, while a male is produced when a spermatozoön without an X chromosome enters the egg. [183] Wilson, E. B., “Studies on Chromosomes,” _Jour. Exper. Zoöl._, 1905, ii., 371, 507; 1906, iii., 1; 1909, vi., 69, 147; 1910, ix., 53; 1912, xiii., 345. “Croonian Lecture,” 1914, _Proc. Roy. Soc._, B. lxxxviii., 333. Such a form is _Protenor_, one of the Hemiptera. Wilson made sure that all the eggs are alike in the number of chromosomes, each egg containing an X chromosome in addition to the six chromosomes characteristic of the species _Protenor_. There are two types of spermatozoa in equal numbers in this species, each with six chromosomes, but one with, the other without, an X chromosome. The two possible chromosome combinations between egg and spermatozoa are therefore as follows (see the diagrammatic Fig. 39): _Egg_ _Spermatozoön_ _Result_ (1) 6 + X + 6 = 12 + X = Male (2) 6 + X + 6 + X = 12 + 2X = Female