242 INTRODUCTION TO CYTOLOGY 



case of chromosome tetrads) the split that is to function in the homceotypic 

 mitosis makes its appearance. The doubleness in the heterotypic pro- 

 phase is therefore not homologous with that in the somatic prophase: 

 in the former it is due to a conjugation and in the latter to a split. 



According to Scheme B the doubleness of the heterotypic prophase is 

 due to a true splitting as in the case of somatic division. In both cases, 

 moreover, the split may have its origin in the preceding telophase. The 

 bivalent chromosome is formed by the association in pairs (often at first 

 end-to-end in the spireme but later side-by-side) of segments of this split 

 spireme at the time of the second contraction. The two split univalents 

 composing the bivalent are separated in the heterotypic mitosis, while 

 in the homceotypic mitosis the separation is along the line of the split 

 originating in the last premeiotic telophase and seen in the spireme of the 

 early heterotypic prophase. The doubleness of the early heterotypic 

 prophase is therefore regarded as homologous with that of the somatic 

 prophase: in both cases it represents a true split. 



It cannot yet be said what the outcome of this controversy is to be. 

 The advocates of Scheme A believe that those of Scheme B have mis- 

 interpreted the changes occurring in the early heterotypic prophase and 

 in all telophases, while the latter charge the former with a neglect of the 

 second contraction stage. Scheme B as fully elaborated by Miss Digby 

 has certain advantages: it allows one interpretation to be placed upon 

 the double spireme in both somatic and heterotypic prophases, irrespective 

 of the exact time at which the split originates, and it also helps to explain 

 the sudden appearance of the split for the second maturation mitosis in 

 the anaphase of the first. Scheme A, on the other hand, is preferred by 

 geneticists because of the earlier and much longer continued association 

 of the conjugating chromosomes, which allows a greater opportunity for 

 " crossing-over" to occur. The significance of this point will be brought 

 out in Chapter XVII. 



This question, however, must be settled primarily by direct evidence. 

 It is obvious that its solution depends upon the exact manner in which the 

 telophasic transformation of the chromosomes and the derivation of the 

 latter from the reticulum in the prophase are accomplished. It is granted 

 by both schools that the alveolar or reticulate condition in which the 

 chromosomes are found in late telophase is continuous with the similar 

 condition seen in the succeeding prophase. If, then, it is true (1) that 

 the telophasic transformation (alveolation) represents a true splitting, 

 and (2) that the early prophasic reticulate condition passes directly 

 into the double spireme, it follows that this doubleness in every prophase 

 is due to the split originating in the preceding telophase. But workers on 

 mitosis are not at all agreed that the evolution of the chromosomes is 

 that stated in (1) and (2). It has been shown in Viciafaba (Sharp 1913), 

 Tradescantia (Sharp 19206), and a number of other instances (see Chapter 



