196 ZOOLOGY FOR MEDICAL STUDENTS chap. 



Now the general occurrence of this longitudinal splitting would appear 

 to carry with it a very interesting logical conclusion namely that the 

 substance of the chromosome is not homogeneous throughout its extent . 

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

 assumption does it become clear why the chromosome splits longi- 

 tudinally — namely in order that every quality spaced out along its 

 course may be equally 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 

 the 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- 

 tures apart from the cytoplasm. They are to be regarded rather as 

 special local modifications of the cytoplasm^ its constituent particles 

 undergoing temporary re-arrangement under the stress of some unknown 

 physical influencCj in somewhat similar fashion to that shown by iron- 

 filings in a magnetic field. The centrosome would appear to be the centre 

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



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

 by far the greater number of clearly-worked-out cases belong to this 

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

 American cytologists. Apart from Insects and Nematodes they have 

 been observed in many other cases scattered through the animal kingdom. 

 The case of Ascaris shows us how sex chromosomes even when present 

 may be unrecognizable through being fused with ordinary chromosomes 

 and 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- 

 istic of the sexual 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 

 gone into in this book. 



The main lessons which the sex chromosome phenomena of Ascaris 

 teach us are these : 



(a) That in the animal in question the microgametes consist in equal 

 numbers of two different classes, 



(b) That these two classes differ from one another in the fact that they 

 produce, when they undergo syngamy, zygotes of opposite sexes, and 



(c) That the male-producing type is characterized by the fact that it 



