II 



MEIOSIS 43 



reticulum the bivalents presently condense out (L, M, N) in the form of 

 long loops, frequently twisted. 



These further condense into " tetrad rods " or double dumb-bells, 

 very similar to many of the bivalents in Lepidosiren. Wilson, however, 

 believes that the condensation takes place in such a way that the trans- 

 verse joints of the bivalents correspond to the original cleft which 

 separates the two components of each diplotene bivalent. ^ If this be so, 

 these transverse joints represent the points of junction of the constituent 

 univalents of each bivalent, and hence they are of quite different origin 

 and significance from the transverse constrictions of the Lepidosiren bi- 

 valents. In accordance with this mode of formation, the transverse joint 

 is the plane of division in metaphase I., which therefore separates entire 

 univalent chromosomes as it does in the two cases already described. 



B. DIVERGENT VIEWS OF THE PROCESS OF MEIOSIS 



While it is generally accepted that meiosis consists in the segregation 

 of homologous chromosomes, it must not be supposed that all cytologists 

 agree that the accounts of the processes leading up to that segregation 

 which have been given above for Tomopteyis, Lepidosiren and Hemiptera 

 are either completely correct interpretations of what occurs in these 

 forms or, even if this were granted, that the process occurs in essentially 

 the same manner throughout the animal (and vegetable) kingdom. 

 However, it is the general scheme of meiosis which is accepted in essentials 

 by more cytologists — at any rate, students of animal cytology — than 

 adhere to any other one scheme, and it is rapidly gaining new adherents. 

 We will now briefly discuss certain other schemes of meiosis that have 

 been proposed, together with a few general problems of this important 

 phase, leaving out of account a few special hypotheses which have been 

 put forward to explain the phenomena observed in certain individual 

 cases, and which can lay no claim to generality. W^e may also leave out 

 of discussion the pure — and, as it appears to the author, unjustifiable — 

 scepticism of certain cytologists (for example, Meves) who deny the 

 possibility of coming at present to any useful conclusion as to how the 

 number of chromosomes is reduced in meiosis. 



(i) Parasyndesis and Telosyndesis - 



These two schemes of meiosis have much in common, and a great 

 measure of generahty is claimed for each by their respective supporters. 



^ Wilson, however, believes that fusion in syndesis is complete, the bivalent chromo- 

 some undergoing internal reconstruction so that it is not possible to homologize the two 

 chromosomes which separate in the diplotene stage with those which united in syndesis. 

 See p. 48. 



^ Called parasynapsis and telosynapsis by cytologists, who employ the term synapsis in 

 the sense in which syndesis is here used (p. 39). 



