366 PROCEEDINGS OF THE OHIO ACADEMY OF SCIENCE. 



sometimes break away or "cross over", to use Morgan's expres- 

 sion, and combine with a character coming from the other parent. 

 Janssens has called attention to the fact that at one stage in the 

 maturation of the germ cells the chromosomes of each pair may 

 be twisted together in a rather close coil and are apparently 

 actually fused at certain points. If in this process the chromo- 

 somes should break transversely at a point of fusion in such a 

 way that a part of each chromosome is transferred to the other, 

 we should have, as Morgan has pointed out, a reasonable physical 

 basis for the phenomenon of ''crossing over". On the other 

 hand, the retention of chromosome identity would give an equally 

 reasonable basis for the phenomenon of linkage. Further it may 

 be assumed that two characters located in adjacent points of a 

 chromosome will be more likely to remain linked than charac- 

 ters widely separated in the same chromosome. After a careful 

 study of the degree of linkage of character with character, 

 Morgan. Sturtevant. Muller, and Bridges, in their newly pub- 

 lished volume on "The Mechanism of Mendelian Inheritance", 

 present a diagram in which they chart the relative positions of 

 some thirty-six unit characters within the four chromosomes 

 of Drosophila. They defend their position in the following 

 words: — "Not only can all the facts of linkage so far studied 

 be explained on this basis, but * * ''' certain further results 

 can be predicted". Hypothetical ; some will say wildly hypothet- 

 ical ; but of fascinating interest, and well worthy of the severe 

 testing that it is sure to receive. 



But, granting that certain characters are carried by the 

 chromosomes in heredity, it does not necessarily follow that all 

 hereditary characters are so carried. Thus Conklin, largely as 

 the conclusion from his cytological studies on the egg, affirms 

 most emphatically that "at the time of fertilization the hereditary 

 potencies of the" two germ cells are not equal, all the early stages 

 of development, including the polarity, symmetry, type of cleav- 

 age, and the pattern, or relative positions and proportions of 

 future organs, being foreshadowed in the cytoplasm of the egg 

 cell, while only the differentiations of later development are in- 

 fluenced ])y the sperm. In short the egg cytoplasm fixes the 



