D. J. SCOURFIELD OX MENDELISM AND MICROSCOPY. 411 



teristic of each species is comparatively small, so that instances 

 are already known in which the characters which " Mendelise " 



are more numerous than the chromosomes. How can cases like 

 these be brought into line with the facts of maturation and 

 fertilisation ? At present there is no definite answer to this 

 question, but the most obvious suggestion is the one made by 

 De Yries and elaborated by Lotsy, that during the conjugation 

 of the chromosomes an interchange of equivalent constituents 

 of each chromosome may take place. Such a process would, of 

 course, have the same effect as increasing the number of chromo- 

 somes, and would therefore give ample scope to the play of 

 Mendelian inheritance amongst the myriad characters composing 

 the complex bodies of the higher organisms. Much more work is 

 wanted, however, to put this and even some of the previously men- 

 tioned ideas about chromosomes on a satisfactory basis, and there 

 are consequently openings here for any number of expert micro- 

 scopists to add to our knowledge of a very important subject. 



Coming to the next point of contact between Mendelism and 

 microscopy — namely, the underlying microscopical basis for many 

 naked-eye characters — we may usefully take as our first example 

 the shape of peas. It will be remembered that in Mendel's 

 experiments the round and wrinkled forms were shown to be 

 distinct unit characters which did not influence one another in 

 any way, except that when they were associated together in a 

 hybrid the wrinkled character remained latent. In every genera- 

 tion pure forms of both types came out in definite proportions 

 from the hybrids, and even after association for six generations 

 the two characters came out just as sharply defined as at first. 

 Apparently Mendel had no particular difficulty in distinguishing 

 round from wrinkled peas, and in the vast majority of cases there 

 is no difficulty ; but it does sometimes happen that a number of 

 seeds, especially in certain varieties, are troublesome to assign 

 definitely to one category rather than the other. Some criticisms 

 of Mendel's law- have, in fact, been based upon this supposed 

 impossibility of placing a certain small proportion of a crop of 

 peas into their proper places. In 1903, however, it was shown 

 by Gregory that round and wrinkled peas are characterised by 

 quite distinct types of starch grains. In the former the starch 

 occurs in ovoid grains with an entire margin, and having 

 frequently a longitudinal internal fissure, in the line of the long 



