156 INTRODUCTION TO CYTOLOGY 



meres then following. It has been pointed out (Sharp 1913) that Muller's 

 figures (Fig. 54, C, D), which are very similar to the later ones of Stras- 

 burger (1907), may be interpreted as steps in the division of a homogene- 

 ous chromatic thread by the formation of vacuoles, and that the 

 chromomeres in this case are merely the cross pieces between the halves 

 of the incompletely split chromosome, as described in the foregoing account 

 of the prophase (Fig. 53, S). 



It is becoming increasingly apparent that the distinction between 

 chromatin granules and supporting thread is not so sharp as has been 

 supposed, since the chromatic substance is often very fluid in consist- 

 ency; and many have felt that the granules when present are far too 

 inconstant in number and behavior to serve as the ultimate units which 

 students of heredity hope to find. On the other hand, it should be said 

 that the constancy in size and position of the chromomeres described 

 by Wenrich (1916) for the grasshopper, Phrynotettix (Fig. 155), argues 

 strongly for the hereditary significance of these bodies, some of which 

 can be seen to retain their identity through the resting stages. But 

 whatever their importance may be, the arrangement of the chromatic 

 material in the form of a long slender thread and its accurate splitting 

 into exactly similar halves are very suggestive in connection with the 

 theory of Roux that many qualities are arranged in a row and all 

 divided at the time of nuclear and cell division. This subject will 

 receive further attention in the chapters dealing with heredity. 



Summary. — The chromosomes, after having arrived at the poles of the 

 achromatic figure, become irregularly alveolized during the telophase and 

 form ragged net-like structures. These are joined to each other by fine 

 anastomoses and so make up the continuous reticulum of the resting 

 stage. In the next prophase this reticulum breaks up into separate 

 small nets or alveolar units, each of which represents a chromosome. 

 The units condense in a peculiar manner and become long slender threads. 

 These threads undergo a longitudinal splitting. The double threads so 

 formed shorten and thicken, and become the double chromosomes which 

 are arranged on the spindle at metaphase. The two halves (daughter 

 chromosomes) making up each double chromosome separate and pass to 

 opposite poles during the anaphase. 



The outstanding and significant feature of somatic mitosis is this: 

 each chromosome is accurately divided into two exactly equal longitudinal 

 halves which are distributed to the two daughter nuclei. The two daughter 

 cells thus receive exactly similar halves of the chromatin of the mother cell. 

 Furthermore, as will be shown below, there is good evidence for the view 

 that the chromosomes maintain an individuality of some sort, so that, 

 since all the nuclei of the body arise by the repeated equational division 

 of a single nucleus, all the somatic (body) cells are qualitatively similar in 

 chromatin content: they contain representatives or descendants of each and 



