102 CYTOLOGY chap. 



a bivalent like the other chromosomes, but are quite separate from one 

 another, and that each is constricted preparatory to division, as if it 

 were a somatic mitosis. Thus we now find six bivalent ordinary 

 chromosomes and two univalent sex chromosomes (only two of the 

 bivalents are shown in this figure). Hence each anaphase group contains 

 both X and Y. In prophase II. X and Y conjugate to form the unequal 

 bivalent shown in Fig. 46, J (metaphase II.), which results in the two 

 kinds of spermatid chromosome groups shown in K and L. 



In still another Hemipteran {Oncopeltus, Wilson, 1912) the X and Y 

 chromosomes are so nearly equal that in many individuals no inequality 

 could be demonstrated, though in others a distinct size difference was 

 detected. Even where they are equal the two sex chromosomes are 

 nevertheless easily identified by their compact form throughout the 

 meiotic prophase. This compact phase is, however, by no means a 

 universal feature of the sex chromosomes, and hence the possibility is at 

 once suggested that forms exist in which there are X and Y chromosomes 

 differentiated physiologically, but not visibly distinguishable from each 

 other or from the other chromosomes. Hence the sexual differentiation 

 of chromosomes, w-hich has been demonstrated for a comparatively small 

 number of animals, so far from being peculiar to them, may be a universal 

 characteristic revealed by the lucky accident that such differentiation is 

 in some animals visible by ordinary methods of microscopic technique. 



We will now consider some other features of the sex chromosomes. 



(2) Which of the two Meiotic Divisions acts as the Reduction Division for 



the Sex Chromosomes ? 



It is a surprising fact that, in the cases just described, the sex 

 chromosomes divide longitudinally in the first meiotic division, while 

 the second is the actual reduction division, separating the X from 

 the Y [Lygaens] or sending the single sex chromosome into the one 

 spermatid and leaving the other spermatid with no sex chromosome 

 {Proienor). In these cases, therefore, the reduction division for the 

 ordinary chromosomes is the first, and for the sex chromosomes the 

 second, meiotic division. What is perhaps even more surprising is that 

 the behaviour of the sex chromosomes varies in this respect in different 

 forms, and sometimes in nearly related species. An example of an insect 

 in which the first division is the differential one is given in Fig. 47. 



The following table, compiled from the exhaustive summary of the 

 numbers of chromosomes in the Metazoa given by Harvey (1917), shows 

 how the orders of insects vary in this respect. It will be noticed that 

 most of the species within any order are alike, but in most orders there 

 are one or two exceptions to the general rule. In compiling this table 



I 



