248 INTRODUCTION TO CYTOLOGY 



pieces, each representing two chromosomes end-to-end; these have the 

 form of loops with a definite orientation ("first boquet stage"). Each 

 segments again, giving the diploid number of split chromosomes, which 

 again assume the form of oriented loops ("second boquet stage"). The 

 halves twist tightly about each other, shorten to form the double bodies 

 seen at diakinesis in the diploid rather than the haploid number, and then 

 conjugate to form the haploid number of chromosome tetrads. The 

 conjugating members (each split) separate at the first mitosis, bringing 

 about reduction; at the second mitosis the separation is along the line of 

 the original split. According to this interpretation, therefore, the double- 

 ness of the early heterotypic prophase is due to a split, as in Scheme B, 

 but the chromosomes arranged end-to-end in the spireme soon become 

 separated and do not conjugate again until diakinesis. 



For a number of years it was thought (Carnoy 1886; Boveri 1887; 

 Hertwig 1890; Brauer 1893) that the chromosome tetrad in Ascaris 

 megalocephala was exceptional in being formed by two longitudinal fis- 

 sions of a primary chromatin rod, there being as a consequence no quali- 

 tative reduction in the two maturation divisions unless the organization 

 of the chromatin were different from that of other organisms. But it has 

 since been shown that they arise as in other organisms by the conjugation 

 of two split chromosomes (Sabaschnikoff 1897; Tretjakoff 1904; Griggs 

 1906). In the oogenesis Griggs reports telosynapsis with prereduction, 

 whereas in the spermatogenesis Tretjakoff describes parasynapsis followed 

 by postreduction. In Ascaris canis (Marcus 1908; Walton 1918) the 

 four chromatids each show a transverse constriction, the chromosomes on 

 the first maturation spindle having the form of octads. 



Although the formation of well differentiated chromosome tetrads 

 occurs very commonly in animals, it appears to be very rare in plants. 

 Farmer (1895) described tetrads in Fossombronia, and they have since 

 been reported in at least three other bryophytes: Pallavicinia (Moore 

 1905), Sphagnum (Melin 1915), and Chiloscyphus (Florin 1918). They 

 have also been described in a few vascular plants : Equisetum (Osterhout 

 1897), Pteris (Calkins 1897), Ariscema (Atkinson 1899), Tricyrtis (Ikeda 

 1902), Thalictrum, Calycanthus, and Richardia (Overton 1909) (Fig. 98), 

 Spinacia (Stomps 1911), Primula (Digby 1912), and Lopezia (Tackholm 

 1914). 



According to Gregoire (1905) such structures in plants are not true 

 tetrads, but resemble them because the chromosomes are often bent and 

 have their material accumulated largely at their ends. Sakamura 

 (1920) interprets them as conjugated constricted chromosomes, and denies 

 that the quadripartite condition has anything to do with reduction in 

 such cases. He likewise accounts for the metasynaptic rod tetrads (Fig. 

 95, E) described by several investigators of maturation in animals, 

 holding that they represent two constricted chromosomes conjugated 



