40 BULLETIN: MUSEUM OF COMPAKATIVE ZOOLOGY. 



B. Division of the Second Spermatocytes. 



At the completion of the first spermatocyte division the two resulting 

 cells have the appearance represented in Figure 39 (Plate 3). The 

 divided centrosomes lie close together and are in close contact with the 

 cell membraue at the pole of the cell opposite the remnants of the spindle. 

 The centrosome is surrounded by no distinct centrosphere, although there 

 is usually a small area of unstained hyaloplasm about it, similar to that 

 seen in the preceding prophase. The astral rays have nearly all dis- 

 integrated, and only a few short indistinct ones still radiate from the 

 centrosomes. 



The nucleus, which is much smaller than in the first spermatocytes, is 

 situated at about one fourth the distance from centrosome to Zwischen- 

 korper. During the second spermatocyte prophase the chromosomes 

 undergo but little change. The most striking thing about them is the 

 fact that the number of chromosomes in the two cells resulting from the 

 same primary spermatocyte is unequal. This inequality of division is 

 due to the peculiar character of the accessory chromosome. As we have 

 seen, this element is derived directly from one of the spermatogonial 

 chromosomes, and during the first spermatocyte prophase possesses a 

 dyad structure, while all of the rest of the chromosomes are tetrads. 

 During the first spermatocyte metaphase this peculiar element may be 

 j'ecognized in most cases, though not in all. 



It is always of the shape represented in Figure 40, chr'so. ace; hut 

 this characteristic alone is not sufficient for its recognition, as the ordi- 

 nary tetrads may also be of this form. However, there is one charac- 

 teristic which, although rather illusive and often difficult of recognition, 

 nevertheless almost invariably serves to differentiate this element from 

 the others. This characteristic consists in the method of its attachment 

 to mantle fibres. The ordinary chromosomes are connected with both 

 poles of the spindle, but the accessory chromosome is connected with 

 only one astral system in this manner. In Figure 40 I have shown this 

 condition as it exists in four cells which are the products of two sperma- 

 togonia. These drawings were made with the greatest care by the aid of 

 a camera-lucida, and only after very careful study of the chromosomes and 

 their relation to the fibres of the spindle. Often the accessory chromo- 

 some is seen to be attached to the one pole by two mantle fibres, and these 

 are inserted into opposite ends of this element. When the rest of the 

 chromosomes divide and separate, during the anaphase, the specialized 

 chromosome passes to one pole undivided, and thus is it brought about 



