SEX-CHROMOSOMES I3 



X-chromosome was first established through the work of 

 Morrill (1910) on the maturation of the egg in various 

 hemiptera, of Boveri (1909) and Gulick (191 1) on the 

 nematode Heterakis and of Mulsow (1912) on the nematode 

 Ancyracanthus cystidicola. In the last of these forms the 

 chromosomes remain separate in the spermatozoon so that 

 they can be counted. The primordial egg cells in the ovary 

 contain 12 chromosomes. During synapsis 6 double ele- 

 ments are to be seen. The first maturation division removes 

 6 complete chromosomes from the egg into the polar 

 body; the second maturation division splits each remain- 

 ing chromosome longitudinally and separates the resulting 

 halves. The mature tgg therefore comes to possess 6 chromo- 

 somes, one of these being the X. 



In the case of the male gamete the primordial germ cells 

 of the testis include only 1 1 chromosomes, one of which is 

 the unpaired X. When these chromosomes conjugate in the 

 synapsis stage the X-chromosome has no partner and it 

 remains apart from the others in the resting nucleus. In the 

 first maturation division there are 5 bivalent chromosomes 

 and the univalent X. When the chromosomes are distri- 

 buted to the daughter cells the X passes undivided to one 

 of these. This heterotypic division yields two primary 

 spermatocytes, one with 5 chromosomes, the other 6. The 

 homotypic division cleaves each of these primary spermato- 

 cytes into two by ordinary mitosis with the result that four 

 spermatids are formed, two of them with 5 and two with 

 6 chromosomes. During their maturation into spermatozoa 

 the chromosomes remain visible and it is possible to observe 

 that the eggs are fertilized by 5 chromosomes and 6 chromo- 

 somes bearing spermatozoa respectively. 



Later cytological work showed that the Protenor type is to 

 be found in most orthopterans, many bugs, beetles, spiders, 

 myriapods and nematodes. Hughes- Schrader (1947), for 

 example, lists 17 species of the bisexual Phasmids which are 

 XO in the male. 



The Lygaeus type is commonly regarded as a more 

 primitive form from which the Protenor type was derived 

 through the progressive loss of the Y-chromosome. It is 



