258 REDUCTION OF THE CHROMOSOMES 



theless rest on strong evidence, and they do not stand in actual con- 

 tradiction of what is known in the copepods and vertebrates. The 

 possibility of such a mode of origin in other forms must, I think, be 

 held open. 



Under the same category must be placed Korschelt's unique 

 results in the egg-reduction of the annelid Ophryotrocha ('95), which 

 are very difficult to reconcile with anything known in other forms. 

 The typical somatic number of chromosomes is here four. The same 

 number of chromosomes appear in the germinal vesicle (Fig. 125, D\ 

 They are at first single, then double by a longitudinal split, but after- 

 ward single again by a reunion of the halves. The four chromo- 

 somes group themselves in a single tetrad, two passing into the first 

 polar body, while two remain in the egg, but meanwhile each of them 

 again splits into two. Of the four chromosomes thus left in the egg 

 two are passed out into the second polar body, while the two remain- 

 ing in the egg give rise to the germ-nucleus. From this it follows 

 that the formation of the first polar body is a reducing division a 

 result which agrees with the earlier conclusions of Henking on 

 Pyrrochoris, and with those of Paulmier on the Hemiptera. 



C. REDUCTION WITHOUT TETRAD-FORMATION 



As already stated (p. 246), the formation of actual tetrads is of 

 relatively rare occurrence, being thus far certainly known only in the 

 arthropods, nematodes, and some annelids. In the greater number of 

 cases the two divisions of the primary chromatin-masses (i.e. of the 

 primary oocyte or spermatocyte) are separated by a considerable inter- 

 val, during which the first maturation cell-division takes place or is ini- 

 tiated, and hence no actual tetrads are formed. This obviously differs 

 only in degree from tetrad-formation, the latter occurring only when 

 the two divisions are simultaneous or occur in rapid succession. 



In the cases now to be considered the length of the pause between 

 the maturation-divisions varies considerably, and in some forms (verte- 

 brates, flowering plants) it is so prolonged that the nucleus is partially 

 reconstructed. In all, or nearly all, these cases the first maturation- 

 division is of the heterotypicalform, the chromosomes having the form 

 of rings and arising by a process that agrees in most of its features 

 with that leading to tetrad-formation. There is here, however, exactly 

 the same contradiction of results as in the case of tetrad-formation 

 described at page 247, and a bewildering confusion of the subject still 

 exists. In brief, it may be stated that most observers of reduction of 

 this type in the lower animals (flat-worms, annelids, mollusks) have 

 found one transverse and one longitudinal division ; most of those 





