50 PHYSICAL BASIS OF HEREDITY 



are present that shorten further into thick rods (Fig. 

 21, g) that often show a single longitudinal split. The 

 Qgg now begins to accumulate the enormous amount of 

 yolk characteristic of selachian eggs ; and during this time 

 the chromosomes become more and more indistinct. As 

 shown in the figure (Fig. 21, h-k) they appear to send out 

 loops laterally, which loops may be only the bendings of a 

 long thread. When the yolk formation is finished the 

 chromosomes condense into shorter threads, with lateral 

 branches (Fig. 21, /). When the Qgg is ripe, the nuclear 

 wall is absorbed, the chromosomes appear as short rods 

 (arranged in twos), which place themselves in the polar 

 spindle. Two polar bodies are given off, leaving the 

 reduced number of chromosomes in the ^g^. 



It is obvious from the preceding account that the sperm 

 and the Qgg pass through essentially the same stages dur- 

 ing maturation, the essential feature of which is the con- 

 jugation of homologous chromosomes followed by their 

 subsequent segregation. Each sperm and each Qgg is left 

 with half the original number of chromosomes — one of 

 each kind. 



Lateral Versus End -to-End FtJSiON of the Chromosomes 



In the preceding account of the union of the chromo- 

 somes only one method of union is described, viz., side-to- 

 side conjugation. The tetrad as represented is due to one 

 division plane between the conjugating pairs, and the other 

 due to a longitudinal split of each conjugating member. 

 But according to some observers, more especially botan- 

 ists, another method of union also occurs, in which the split 

 chromosomes unite end to end. If the division planes 

 in such a tetrad represent respectively the plane of union 

 at the ends, and the longitudinal split through the united 

 rods, the final result of this separation would be exactly 

 the same so far as the four elements of the tetrad are 

 concerned, but the process would have serious conse- 



