THE STRUCTURE OF CELLS 31 



longitudinal halves, becomes specially aggregated at two spots in 

 each. Thus are formed the so-called Tetrads, to which much im- 

 portance has been attached in the theoretical interpretation of the 

 whole process of reduction. For it is thus seen that in the above 

 cases the tetrad may be regarded as having originated first by a 

 longitudinal fission of the chromosome rudiment, and then by a 

 transversely isolated aggregation of chromatin in each half. 



It may also happen that tetrads are formed in a manner less 

 easy to follow out, as in Helix and in Arion, in which the separate 

 filaments are difficult, if not impossible, to trace in the stages 

 immediately preceding their formation ; the chromatin thus appear- 

 ing, so to speak, to travel to and become aggregated at definite 

 areas, and to assume in a somewhat irregular manner the ring form 

 of tetrad, similar to that occurring in Gryllotalpa, as described and 

 figured by vom Rath. 



Still another type of tetrad formation has been described by 



FIG. 1U. 



Diagram illustrating tetrad formation. A, the split thread (spirem) stage. /?, later stage, 

 showing aggregation of chromatin at each end of the split bivalent chromosomes. C', fully 

 formed tetrads, of which the one to the right represents the most typical form. 



Brauer as occurring in the spermatogenesis of Ascaris megalocephala 

 already alluded to above. The lining filament first contains a single 

 row of chromatin granules, each of the latter divides crosswise into 

 four, which lie in the same transverse plane, and hence the original 

 filament now contains four rows of chromatin granules. As the 

 process of shortening and thickening progresses, these become, so 

 to speak, telescoped together, and the end view of each filament 

 exhibits four chromatin masses corresponding to the four rows 

 just described, and which thus appear as tetrads similar to those 

 of Cyclops, although they would appear to have a very different 

 origin. For whereas in the latter case the single units of the 

 tetrad have arisen as the consequence of two longitudinal fissions 

 of the original chromatin granules, in the case of Cyclops the same 

 appearance is apparently produced partly by a longitudinal fission, 

 and partly by a transverse delimiting of the original granules. In 

 the first maturation division of the egg of the same animal, each 

 chromosome is seen to be divided completely into four segments, 



