I9 6 i LOGY FOR MEDICAL STUDENTS CHAP. 



,il on -urrcncc of this longitudinal splitting would appear 

 v with it a vrry interesting logical conclusion namely that the 

 i tin- chromosome is not homogeneous throughout its extent 

 I ut differs in quality from point to point along its length. Only on this 

 assumption docs it become clear why the chromosome splits longi- 

 tudinallynamely in order that every quality spaced out along its 



[iially shared between the two daughter chromosomes. 

 In relation to the phenomena of heredity we appear then to be justified 

 in the belief that not only are hereditary qualities in general carried by 

 tin' chromosomes but that different hereditary qualities are localized in 

 different portions of the individual chromosome. 



(7) In the description of the mitotic process reference has been made 

 to the centrosomes and to the fibrils which constitute the spindle. The 

 student should guard himself against regarding these as discrete struc- 



ipart from the cytoplasm. They are to be regarded rather as 



local modifications of the cytoplasm, its constituent particles 



t emporary re-arrangement under the stress of some unknown 



,il influence ; in somewhat similar fashion to that shown by iron- 



i n a magnetic field. The centrosome would appear to be the centre 



trom which this influence, whatever it may be, radiates. 



(8) Sex chromosomes were first observed in insects and even to-day 



er number of clearly-worked-out cases belong to this 



group of animals, most of our knowledge having been accumulated by 



American cvtolouists. Apart from Insects and Nematodes they have 



"1 in many other cases scattered through the animal kingdom. 



'se of Ascaris shows us how sex chromosomes even when present 



unrecognizable through being fused with ordinary chromosomes 



an<l we may take it as probable that a similar explanation accounts for 



their unrecognizability in other types where they appear to be absent 



and that the presence of such chromosomes is really a general character- 



tlie >e\iial differentiation of gametes. 



In cases where sex chromosomes are definitely known to occur they 

 exhibit differences in detail, in numbers and so on, which need not be 

 into in this book. 



ons which the sex chromosome phenomena of Ascaris 



I liai in the animal in question the microgametes consist in equal 

 miml.er.s n| two different classes, 



differ from one another in the fact that they 

 hen they undergo s\ n u amy, zygotes of opposite sexes, and 



ile- producing type is characterized by the fact that it 



