26O BERTRAM G. SMITH. 



plate, and the observations of Morris (1914) are in agreement. 



Metz (1916) has found in the diptera a pairing of homologous 

 chromosomes, simulating synapsis, which occurs in all tissues, 

 somatic as well as germinal; this association of maternal with 

 paternal chromosomes was found in late cleavage and during 

 all later stages of embryonic development. From the following 

 statement by Overton (1909) we have a suggestion of a similar 

 occurrence in plants, though attention is directed more particu- 

 larly to the matter of genetic continuity of individual chromo- 

 somes: "In the somatic nuclei (of certain plants) the chromo- 

 somes are represented during rest by definite visible bodies, the 

 pro-chromosomes, which are arranged in parallel pairs, with 

 apparent linin intervals. These heterogeneous spirems, the 

 homologous portions of which have early become associated in 

 pairs, probably remain distinct throughout the life-history of the 

 sporophyte." 



In spite of the mingling and even paired association of maternal 

 and paternal chromosomes, there are reasons for believing that 

 the two kinds of chromosomes maintain their independence 

 until gametogenesis. This leads us to a consideration of the 

 doctrine of the genetic continuity of individual chromosomes, 

 which goes further than the principle of duality of the embryonic 

 nuclei, but confirms it as a universal law. 



The remarkable constancy in the number of chromosomes 

 throughout the cells of a given organism and species has long 

 been known, and affords important evidence for the view that 

 the chromosomes are persistent as individual structures. To 

 be sure, it sometimes happens in mitosis that one or more chromo- 

 somes belonging to one daughter-group, accidentally become 

 included with the other group so that one of the daughter-nuclei 

 has fewer, the other more, than the normal somatic number; 

 but such an occurrence is very exceptional, and in subsequent 

 divisions of these cells the number of chromosomes appearing is 

 not the normal, but the increased or diminished number (Boveri, 

 1890). Whatever the number of chromosomes entering into the 

 formation of a resting nucleus, the same number afterwards 

 issues from it. 



