152 M. \V. r.LACKMAN. 



mosome N shows a considerably different shape, corresponding 

 to its different history. It is shorter and in one plane broader 

 than the other dyads derived from the double-rod-shaped tetrads 

 of the first spermatocyte. It has been already shown that chro- 

 mosome N of the first spermatocyte probably undergoes a trans- 

 verse division while the other tetrads are dividing longitudinally, 

 and we would, therefore, expect the products of this division to 

 present a different appearance from the other dyads. As a 

 matter of fact, it is of quite a different shape from the others 

 derived from the double-rod tetrads. 



The differences in size between the various chromosomes of 

 the different types is, of course, only half as great in the second 

 spermatocyte, as it is in the first spermatocyte, and therefore 

 there is not such certainty in identifying the various individuals 

 of the different types. But the same size ratio seems to exist 

 and the chromosomes of the different types can readily be ar- 

 ranged in a graded series as regards size, just as in the first 

 spermatocyte. 



It has been shown by this study that the chromosomes of 

 Scolopendra heros cannot be considered as ephemeral structures, 

 which have one appearance in one cell and present an entirely 

 different form in another cell of similar history. Any study 

 except a very superficial one must lead to an entirely different 

 conclusion. By a study of many hundreds of cells in various 

 stages of mitosis it has been found that the number of chromo- 

 somes in the primary spermatocytes is absolutely constant and 

 invariable. Furthermore, these chromosomes show other char- 

 acteristics, which speak very strongly for their individuality. 

 The ordinary chromosomes arc divisible into three types on the 

 basis of the shape they assume in the prophase and metaphase of 

 the first maturation division, and in their relation to the mantle 

 fibers of the spindle. The individuals of each type of structure 

 are invariably of the same number and in all favorable cases each 

 chromosome of a given type is distinguishable from the others of 

 a similar shape by a difference in size. 



In addition, several of the chromosomes possess certain in- 

 dividual peculiarities aside from shape and size, which serve 



