THE MORPHOLOGY OF FERTILIZATION 71 



The exact facts about the origin of the sperm aster 

 in fertilization should therefore be most carefully ascer- 

 tained. In the sea urchins, where the process was first 

 carefully studied, the aster begins to be visible shortly 

 after the rotation of the sperm head has begun. It is 

 focused at the base of the sperm head, thus in the 

 region of the middle piece; the latter is not, however, 

 in the center, but to one side, of the aster, as Meves's 

 very detailed study shows (cf . Fig. ye) . A differentiated 

 centrosome is not demonstrable in the center of the 

 forming aster (Fig. 7^), or at most, as Boveri says, it 

 is such an immeasurably small granule that it can be 

 seen only in especially favorable cases, and then only 

 because of its position; it is never to be seen prior to 

 the origin of the aster. In Nereis, in which the middle 

 piece of the sperm does not enter the egg, the sperm 

 aster appears at first to be focused at the base of the 

 sperm nucleus itself and only by degrees separates 

 from it and acquires a distinct centrosome (Figs. 8a, Z>, c). 

 I have, moreover, been able, by application of a strong 

 centrifugal force to the inseminated eggs, to remove not 

 only the middle piece but also variable parts of the base 

 of the sperm head itself before penetration, so that a 

 reduced sperm nucleus forms after penetration. In the 

 case of such nuclei asters form at the base, opposite to 

 the perforatorium, in the usual way, which are more or 

 less proportional in size to the nuclear fragment concerned 

 (Fig. 11). This shows that the sperm nucleus itself in this 

 case has the capacity to induce localized aster formation 

 in the egg cytoplasm; this reaction might be conceived 

 to be due to a specific centrosome substance contained 

 within the nucleus, but for this there is no evidence. 



