IMPORTANCE OF THE SPERM ASTER 261 



giving origin to the first cleavage amphiaster, whereas the sperm possessed a 

 dynamic centrosome with its included centriole but lacked sufficient cytoplasm 

 for division or cleavage. Consequently, fertilization brought together a rela- 

 tionship necessary for cleavage and development. This idea was first set forth 

 by Boveri (see fertilization theories at the end of this chapter). 



In the majority of animals, the central body (i.e., the centrosome) with 

 its surrounding aster, which ultimately divides and gives origin to the first 

 cleavage amphiaster, does not arise until after the sperm has entered the egg. 

 In these cases the aster complex arises in the middle piece of the sperm in 

 close proximity to the nucleus. These facts are well illustrated in figures 116, 

 117, and 131. Many studies of the fertilization process and early cleavage 

 bolster this general conclusion. There are some exceptions, however, to this 

 rule. For example, Wheeler ( 1 895 ) in his studies of fertilization in Myzostoma 

 glabrum demonstrated that the centrioles of the egg near the germinal vesicle 

 give origin to the amphiaster concerned with polar body formation. Following 

 the maturation divisions, the female pronucleus with its centrioles and forming 

 amphiaster, migrates along the copulation path to meet the sperm pronucleus. 

 The amphiaster and centrioles are closely adherent to the egg pronucleus 

 during the migration of the latter. In the honeybee, Nachtsheim ('13) found 

 a similar situation, while in the mollusk, Crepidula plana, Conklin ('04) found 

 evidence which suggests that one aster of the cleavage amphiaster arises from 

 the egg, whereas the other aster arises from the sperm, "although there is 

 not positive evidence that they are directly derived from egg and sperm 

 centrosomes." 



Where the egg develops as a result of artificial stimulation the first cleavage 

 spindle arises without the aid of the sperm middle piece. In these instances 

 the amphiaster probably is derived from the central body of the last matura- 

 tion division, or, it may be, from certain asters or cytasters artificially induced 

 in the egg cytoplasm by the activation process. The production of numerous 

 asters in the cytoplasm of the egg by artificial stimulation has long been 

 known (Mead, 1897; Morgan, 1899, '00). 



The general conclusion to be extracted from the evidence at hand, there- 

 fore, suggests that the central body from the last maturation spindle or other 

 artificially induced asters in the egg cytoplasm may form the amphiaster of 

 the first cleavage spindle in the case of an emergency. Such an emergency 

 arises in normal parthenogenesis or in cases of artificial activation (artificial 

 parthenogenesis) of the egg. However, under the conditions of normal fer- 

 tilization the sperm aster fulfills the role of developing the first cleavage 

 amphiaster. 



Regardless of the fact that the first cleavage amphiaster appears to be de- 

 rived from the middle piece of the sperm, the influence of the egg protoplasm 

 is undoubtedly an important factor in its formation. In the normal polyspermy 

 of the newt, Triton (fig. 138; Fankhauser, '48), the sperm aster nearest the 



