464 



THE EEPEODUCTIVE SYSTEM 



termed the Jiaploid group. The loops now lose their polarized condi- 

 tion (postsynaptic phase) and become scattered throughout the nucleus, 

 which has meanwhile grown in size, and become more or less closely 

 united into a more or less continuous thread. This soon shows evidences 

 of a longitudinal split (Fig. 12), the diplotene phase; subsequently 

 this undergoes transverse segmentation into half the original number 



(23) of chromosomes, irrespective of the ac- 

 cessory; thus there are 11 segments. The 

 nucleus contains in addition a pale-staining 

 plasmosome, and the deep-staining accessory 

 chromosome in its characteristic location. 

 Figs. 11 to 14 are designed to show these 

 two structures in varying relationships. Fig. 

 15 illustrates a late prophase condition, in 

 which the euchromosomes (McClung) are 

 less compact than the heterochromosome 

 (accessory), and in the typical tetrad or 

 quadripartite condition. 



At this point we may recapitulate our 

 facts regarding the chromosomes. The sper- 

 matogonium contains 23 (Fig. 423 A). 

 With one exception the chromosomes can be 

 grouped in pairs; such grouping gives 11 

 pairs. Moreover, the chromosomes can be 

 arranged in a progressive series from the 

 viewpoint of size; the unpaired, or accessory 

 chromosome (lOx) takes rank between the 

 9 and 11 chromosomes. These chromosomes 

 represent the original contribution from 

 father and mother when the egg was fertilized preceding development. 

 At synapsis the chromosomes actually pair and fuse; and obviously in 

 the manner indicated in (A), for the chromosomes can again be ar- 

 ranged in a graded series, corresponding approximately in size and range 

 to the pairs indicated. Since the accessory has no mate the reduced 

 number of chromosomes is 12. 



FIG. 422. PRIMARY SPERMA- 

 TOCYTE OF A GRASSHOPPER, 

 HlPPISCUS TUBERCULATUS, 

 SHOWING THE COMPACT 

 ACCESSORY CHROMOSOME 

 (x) AMONG THE PALER 

 MOSSY PROPHASE EUCHRO- 

 MOSOMES, AND THE IDIO- 



SOME (l). 



The cytoplasm contains 

 many granular and filamen- 

 tous mitochondria. Flem- 

 ming fixation; iron-hematox- 

 ylin stain. X 1800. 



It is now quite generally believed that the chromosomes of a pair rep- 

 resent paternal and maternal contributions; thus from the viewpoint of 

 inheritance, assuming that the chromosomes are the vehicles of certain 

 hereditary characters and qualities, the offspring of a pair of parents inherit 



