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THE AMEBIC AX NATURALIST [Vol. XLIX 



thought to represent the resting nuclear condition of the 

 chromosomes. Prochromosomes have been found in at 

 least sixty species of plants, and various structures com- 

 parable to them in many others. These investigations 

 favor the thought that the chromosomes are persistent 

 morphological entities; nevertheless they are not suffi- 

 cient to establish the matter if there were no other data 

 at hand. 



There is a series of facts, however, which is more con- 

 vincing. We are told that in addition to each species of 

 animal or plant having in the larger part of its cells a spe- 

 cific number of chromosomes, there is a constant reap- 

 pearance of the different shapes and sizes of these chro- 

 mosomes in the same positions relative to one another 

 during cell division after cell division. 



Strasburger says: "The observation of such a series 

 of stages of nuclear division as can be obtained by the 

 laying open of embryo sacs in which development of 

 endosperm tissue is commencing, makes it difficult to re- 

 sist the impression that it is always the same chromo- 

 somes which make their appearance over and over again 

 in the repeated divisions. In the prophase, the chromo- 

 somes are seen to appear in precisely the same position 

 that they occupied in the preceding anaphase, and if the 

 picture of the anaphase were proportionally enlarged, it 

 would exactly correspond to that of the succeeding pro- 

 phase." 



The facts from which these general conclusions have 

 been drawn can not be denied. Baltzer found odd-shaped 

 chromosomes of similar shape in many maturing eggs of 

 sea urchins. Boveri, Montgomery and later Schaffner 

 pointed out a constant difference in the form and the size 

 relations of the two chromosomes of Ascaris megalo- 

 cephala univalens. Sutton thought he could recognize 

 each individual chromosome in eleven consecutive cell 

 generations of the maturing germ cells of the lubber 

 grasshopper Brachystola magna. The so-called sex chro- 

 mosome which has been found in so many insects and 



