54 CYTOLOGY CHAP. 



(4) Which of the two Divisions of the Meiotic Phase effects the 

 Separation of the Homologous Chromosomes P 



Korschelt and Heider, in their general account of meiosis (1903) give 

 two subdivisions of the different modes of meiosis which they recognize. 

 These are called Pre-reduction and Post-reduction according as to whether 

 it is the first or second division of the meiotic phase which effects the 

 reduction (by separating homologous chromosomes). 



In Tomopteris and Lepidosiren it is plain that it is the first division 

 which does this, and the great majority of modern accounts of meiosis agree 

 in this. They therefore fall into the category of pre-reduction. Most 

 of the earlier accounts; however, favoured post-reduction, but this was 

 mainly due to those errors regarding the composition and fate of tetrads 

 of the Copepod type which have been explained above, and may therefore 

 be ignored. 



As will be readily appreciated, it is often very difficult to decide 

 which of the two divisions effects the reduction, especially in the case of 

 tetrads formed, as in Ascaris, by two longitudinal planes. Here it is a 

 question of tracing the two planes without break from their inception 

 into metaphase I. in order to see which plane it is that is operative in 

 this division. 



Although pre-reduction appears to be by far the commoner, post- 

 reduction has been quite definitely described by competent workers, 

 especially in certain insects. Indeed in this group it appears that even 

 in the same animal some bivalents may divide reductionally in the first 

 and others in the second division (M'Clung, 1914). In the case of certain 

 peculiar chromosomes, the " sex chromosomes " (to be described in 

 Chapter IV.), no fact in cytology is better established than that these may 

 undergo pre-reduction in some species and post-reduction in others (p. 102). 



(5) Synizesis 



The significance of this very characteristic stage in meiosis is quite 

 obscure. The one thing certain is that it can have no direct necessary 

 connection with syndesis, since in many forms (e.g., Tomopteris) it is 

 absent. The fact that it is not found in certain species has even led some 

 cytologists, who happen to have worked chiefly with such species, to 

 doubt its natural occurrence at all, and to ascribe the presence of it in 

 forms used by other workers to their faulty methods of fixation. In 

 many organisms, however, synizesis occurs whatever method of fixation 

 is used, and the matter appears finally settled by the observation of 

 synizetic contraction in living and fresh tissues. A few examples out of 

 many such observations are those of Wilson (1909 b) in Anasa, Arnold 

 (1909) in Planaria, Schleip (1909) in Ostracoda. 



