194 COOK 



reproductive subdivisions occur. It is not until the sex-cells for 

 the next generation are to be formed that conjugation is resumed 

 and brought to a conclusion by mitapsis.^ 



Among the lower groups new organisms are built up at this 

 stage, after conjugation has been completed by mitapsis, and 

 before a new conjugation begins, as in the mosses, liverworts, 

 and lower algas and fungi. But the higher plants and animals 

 do not attempt to build up structures of these simple cells which 

 are the products of completed conjugations. Instead of forming 

 cellular structures in the intervals between conjugations, the 

 process of reproduction in the higher types hastens from one 

 conjugation to another, as though the superiority of the double 

 cells for structure building were clearly recognized. 



The ferns may be cited as an example of a group in which 

 two different structures are built. A little liverwort-like pro- 

 thallus is formed after conjugation, and a large leafy structure, 

 or frond, during conjugation. Flowering plants, however, have 

 closely approximated the reproductive methods of the higher 

 animals. They do not waste time in structure-building cell- 

 divisions after or between conjugations, but pass promptly from 

 one conjugation to another. 



Applying these facts to our experiment in Mendelism, we find 

 that the process of reproduction, as conducted by the higher 

 plants and animals, does not permit of a complete conjugation 

 before the building up of the new organisms which we have been 

 describing as the first generation of the cross we intended to 

 make. Another discrepancy between words and facts has to be 

 admitted. Just as we found ourselves unable to cross the origi- 

 nal parents, and were obliged to content ourselves with the in- 

 tention of combining gametes produced by those parents, so we 

 have now encountered a new limitation, inherent in the methods 

 of reproduction followed by the higher plants and animals. 

 We are unable to bring about an immediately complete union of 



'The third and final stage of the process of conjugation lias been named mi- 

 tapsis from the Greek words meaning" thread-fusion, because the chromatin 

 granules, supposed to represent the two original parents, are, at least in some 

 striking cases, stretched out in long threads. This permits them to fuse in pairs 

 and thus to achieve a much more thorough and complete union than hy any less 

 definite method of combination. 



