212 THOMAS H. BUYG1<;. 



here complete, and double rods were formed, which divided 

 transversely, and then united again by their ends, so that the 

 original tetrad figure was replaced by a pair of rods lying 

 side by side. Riickert and Haecker, in explanation of their 

 results, adopted the "apparent reduction" hypothesis,— that 

 is, the reduction is only apparent in the first division, and 

 is realised in the second, by a suppression of a second 

 longitudinal splitting. It is to be ])articularly noticed that 

 in Cyclops Haecker makes the chromosomes of the second 

 division bivalent. 



It is interesting to note that, except in insects, the type, 

 which Haecker calls the plant type, has claimed most of 

 recently described cases. In this type, as before explained, 

 the typical tetrad formation is absent, and Haecker homo- 

 logises the rings described in the prophases with the tetrad 

 groups by making them equivalent to the four elements of 

 these bodies. Griffin has attempted to establish the same 

 analogy by imagining his cross-figures as derived from a 

 crushed ring, the four limbs of which represent the four 

 bodies in the tetrad. 



Echinus will not fall into any of Haecker's types. The 

 special value of the observations in Echinus seems to me that 

 the heterotypical division is present without previous ring 

 formation on the one hand, and on the other distinct tetrads 

 are formed which certainly submit to a second longitudinal 

 division. 



I shall now endeavour to explain my results in terms of the 

 tetrad, bub 1 must first of all refer to Boveri's and to Wilson's 

 figures of the second polar spindle in Echinus niicro- 

 tuberculatus and Toxopneustes. They both show obvious 

 dyads in the equatorial plate, exactly as seen in Ascaris and 

 Gryllotalpa, and this was the interpretation I was at lirst 

 inclined to give to the appearances, until I convinced myself 

 of the compound nature of the bodies, which at once trans- 

 ferred Echinus from the group represented by the insects to 

 that represented by tlie inore recently described case of the 

 Turbellarians, that is to Haecker's plant type, though, as I 



