1 08 CELL-DI J 'I SI ON 



astral rays sometimes extending throughout almost the entire Qgg^ 

 only to disappear or become greatly reduced without the occurrence 

 of division, the ensuing cleavage being effected by a new am})hiaster 

 or by the recrudescence of the old.^ For these and other reasons we 

 must admit the probability that contractility of the astral fibrillx', if 

 it exists, is but the expression or consequence of a more deeply lying 

 phenomena of more general significance. The subtlety of the prob- 

 lem is strikingly shown by Boveri's remarkable observations on 

 abnormal sea-urchin eggs ('96), which show (i) that the periodic 

 division of the centrosome and formation of the amphiaster may take 

 place independently of the nucleus; (2) that the spindle, as well as 

 the asters, is concerned in division of the cell-body ; and (3) that an 

 amphiaster without chromosomes is unable to effect normal division 

 of the cell-body. The first and third of these facts are shown by eggs 

 in which during the first cleavage all of the chromatin passes to one 

 pole of the spindle, so that one of the resulting halves of the tgg 

 receives no nucleus, but only a centrosome and aster. In this half 

 perfect amphiasters are formed simultaneously with each cleavage in 

 the other half, yit no division of the protoplasmic mass occurs? The 

 second fact is shown in polyspermic eggs, in which multipolar astral 

 svstems are formed by union of the several sperm-asters (Figs. 53, loi ). 

 In such eggs cleavages only occur between asters that are joined by a 

 spindle. Normal cleavage of the cell-body thus requires the complete 

 apparatus of mitosis, and even though the fibres be contractile they 

 cannot fully operate in the absence of chromatin. 



We may now turn to theories based on the hypothesis, first sug- 

 gested by Fol in 1873, that the astral foci {i.e. centrosomes) represent 

 dynamic centres of attractive or other forces. It should be noted that 

 this hy])othesis involves two distinct questions, one relating to the 

 origin of the amphiaster, the other to its mode of action ; and we have 

 seen that some of the foremost advocates of the contraction-hypothesis, 

 including Van Beneden and Boveri, have held the centrosomes to be 

 attractive centres. Apart from the movements of the chromosomes, 

 the most obvious indication that the centrosomes are dynamic centres 

 is the extraordinary resemblance of the amphiaster to the lines of force 

 in a magnetic field as shown by the arrangement of iron-filings about 

 the poles of a horseshoe magnet — a resemblance pointed out by Fol 

 himself, and urged by many later writers,'^ especially Ziegler ('95) 



ir/:p. 213. 



' This result is opposed to Boveri's earlier work on Ascaris (p. 355), and is modified by 

 Ziegler ('98), who observed in a single case that an irregular cleavage occurred in the 

 enucleated half after two or three divisions of the centrosome. On the other hand, it is sup- 

 ported by Morgan's convincing experiments on the eggs oi Arbacia (p. 308). 



2 Cf. the interesting photographic figures of Ziegler ('95). A still closer simulacrum of 

 the amphiaster is produced by fine crystals of sulphate of quinine (a semiconductor) sus- 



