50 CYTOLOGY chap. 



division of each one of these smaller elements, and hence the two daughter 

 chromosomes derived by the longitudinal fission of the mother chromo- 

 some are identical. If, however, a chromosome were to divide trans- 

 versely, the two resulting chromosomes would be dissimilar, as each 

 would contain only a portion (half) of the smaller elements. 



Now in a meiosis with tetrad formation of the Copepod type it follows 

 that if both the joints represent division planes, one division must be longi- 

 tudinal (or equational, since the resulting daughter chromosomes receive 

 similar sets of chromatin elements), and the other division must be 

 transverse (or rediictional, since each resulting daughter chromosome 

 receives only one-half of the set of chromatin elements). As we have 

 seen, however, it was by an error of observation that the transverse 

 joint in a Copepod tetrad was taken to be a division plane, and it is now 

 almost universally held that " reduction " consists in the separation of 

 entire homologous chromosomes, and not in their transverse division. 

 Indeed, while there are many cases, notably amongst insects, still in 

 need of elucidation, it is more than doubtful if transverse division of 

 chromosomes in the above sense ever occurs as the result of mitosis. 



As mentioned above, the " tetrads " of Ascaris megalocephala consist 

 of four masses, one of which eventually reaches each of the four sperma- 

 tids. In other words, both joints of the tetrad are division planes. 

 An examination of the mode of formation of these tetrads reveals, how- 

 ever, that they are constituted differently from that which we have called 

 the Copepod type, as exemplified in Lepidosiren, etc. 



Ascaris has been the subject of a very great number of cytological 

 investigations. The classical description of the meiotic phase (in the 

 male) is that given by Brauer (1893) for the variety A. m. bivalens, in 

 which, it will be remembered, the diploid chromosome number is four. 



His description practically starts with synizesis, in which the chromatin 

 is contracted to one side of the nucleus in a fairly compact mass, from 

 which, however, chromatin threads project (Fig. 20, A). These threads 

 are at once seen to be double, and when cut in transverse section, or seen 

 in end view, they reveal themselves as quadruple, being divided longi- 

 tudinally by two division planes at right angles to one another (Fig. 

 20, B). These quadruple threads are to be interpreted as formed by 

 syndesis of two homologous chromosomes, each split longitudinally (in ' 

 preparation for the second division of the meiotic phase). Brauer, 

 whose investigations were carried out before the modern ideas as to 

 syndesis had been formulated, did not interpret the quadruple threads 

 thus, but as formed by the double splitting of a single thread. Later 

 work on meiosis in this animal, however (de Saedeleer, 19 12), has revealed 

 all the principal stages as found in Tomopteris. 



