II MEIOSIS WITH TETRAD FORMATION 49 



On the other hand, the view that organic continuity is estabhshcd 

 between the two participating chromosomes is much more securely 

 founded on the very numerous cases where complete contact is observed 

 between the constituents of each pachytene thread, either over the whole 

 length of the thread at once, or now at one point and now at another. 



(3) Meiosis with Tetrad Formation 



Much error and confusion has been introduced into the study of the 

 meiotic phase by the failure to recognize the true nature of the transverse 

 constrictions of the meiotic chromosomes, and that they are often of 

 no direct significance in the process of reduction. It was for long supposed 

 to be a general rule that the quadripartite chromosomes, so commonly 

 formed in meiosis by the junction of two bipartite chromosomes, are 

 composed of four masses, one of which eventually reaches each of the 

 four spermatid nuclei. These " tetrads," as they were called, were 

 supposed to divide across one joint in the first meiotic division, giving 

 two " dyads " — one to each spermatocyte II. In the second division 

 the dyads were supposed to divide across at the remaining joint, giving 

 one " monad " to each spermatid. Thus one division of the tetrad was 

 said to be longitudinal and one transverse. 



In the case of Ascaris megalocephala, the organism to which our 

 knowledge of cytology is due probably more than to any other one 

 species, this is indeed the fate of the tetrads. These are, however, pro- 

 duced in this species in a different way from that described iovLepidosiren, 

 as will be shown below. A similar partition of the four parts of the 

 tetrads among the four spermatids has been frequently described for 

 insects also. Here again it appears that the apparently transverse 

 constriction has an origin different from that in the lung-fish (p. 43). 



In other cases, however, notably the Copepoda, accounts of the 

 distribution of the four segments of each tetrad, one to each spermatid, 

 were based on faulty observation. In this group it has more recently 

 been shown (Lerat, 1905 ; Matschek, 1910) that the chromosomes which 

 separate in anaphase II. are not monads formed by division of the dyads 

 into their two parts, but are still bipartite, being formed by longitudinal 

 division of the dyads of anaphase I. Thus the transverse joints of the 

 Copepod tetrad are of the same nature as those in Lepidosiren, and do 

 not represent a division plane. 



The false view of the composition and fate of what may be called the 

 Copepod type of tetrad was intimately connected with the older theories 

 of meiosis. At an early period of cytological theory (Roux, 1883 ; Weis- 

 mann) it was recognized that a chromosome consists of a number of 

 smaller dissimilar elements arranged in linear series (see Chapter V.). 

 Consequently, a longitudinal division of the chromosomes results in the 



E 



