52 CYTOLOGY 



CHAP. 



illustrated by Fig. 20, L-P, which show the condensation of the tetrads 

 in nuclei in which the parts of each bivalent are well separated. 



Each bivalent tetrad therefore now consists of four parallel rods 

 produced by the longitudinal apposition of two homologous univalent 

 chromosomes each longitudinally divided into two daughter chromosomes. 

 Before the metaphase is reached each of the four rods has so contracted 

 as to be nearly spherical, and the tetrad now consists of four nearly 

 spherical masses, one of which, as is sufficiently explained by the figures, 

 is distributed to each of the four spermatids. Thus, as in Tomopteris 

 and Lepidosiren, one of the two meiotic divisions is reductional in the 

 sense of separating homologous chromosomes, and the other, Uke an 

 ordinary mitosis, separates the daughter chromosomes produced by 

 fission of the mother chromosome. WTiile there is here no means of 

 deciding which division does which, it may be assumed from analogy 

 that the first division is the reductional one. 



The resemblance between the " tetrads " of Ascaris and those formed 

 by some of the chromosomes in Lepidosiren or in the Copepoda is very 

 close, but obviously only superficial. In each case the tetrad is bivalent, 

 but in the one case [Ascaris) the joint in each univalent is really longi- 

 tudinal (as shown by its origin) and marks the plane of division of the 

 second meiotic division. In Lepidosiren and the Copepoda, on the other 

 hand, the joint in each univalent is a mere transverse constriction, and is 

 not operative as a division plane. 



The accompanying diagram illustrates the structure of a meiotic 

 chromosome in Lepidosiren, Ascaris, and an insect such as Oncopeltus. 

 The Hnear series of dissimilar elements of which the chromosome is com- 

 posed is represented by the letters of the alphabet, one chromosome being 

 represented by A B C D and its homologue by abed. The diagram 

 illustrates the fact that the first and last stages (syndesis and gametes) 

 are alike in all three cases, and also that in all three it is the first meiotic 

 division which separates the homologous chromosomes. 



The early appearance of the longitudinal fissure which becomes 

 operative in the second division, and which is responsible for the " tetrad " 

 form of the Ascaris bivalent, is a common feature of meiosis. WTien both 

 this fissure and a transverse joint are present in each component of the 

 bivalent, the chromosome assumes an " octad " shape. This is well 

 exempUfied in the bivalents of certain Copepods. 



It would take us too far to consider other schemes of meiosis which 

 have been proposed, especially as none of them can establish any claim 

 to general application. A warning, however, is necessary against the 

 too ready acceptance of accounts in which no syndesis proper is said to 



