STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 67 



by the fertilizing spermatozoon; in others, the egg and spermatozoon 

 are each thought to contribute a centriole. In eggs beginning 

 parthenogenetic development, the aster forms after division of a 

 centrosome that may have persisted from the second maturation 

 spindle or may have been generated de novo in the cytoplasm. The 

 nuclear sap evidently contributes something to the formation of the 

 spindle, so that the division apparatus is normally both cytoplasmic 

 and nuclear in origin, and predominantly the former. Under certain 

 experimental conditions, however, supernumerary asters (cytasters) 

 can be induced in invertebrate eggs (Wilson, 1928) and some 



Fig. 55 

 Early telophase, first-meiotic spindle (rat). The intermediary 

 body is very distinct. X 2,000. 



Fig. 56 

 Metaphase second-meiotic spindle 

 in a field- vole egg. x 1,500. 



cleavage with cytasters has been seen in enucleated egg fragments 

 (Harvey, 1936), so that an active division apparatus can be formed 

 without direct nuclear contribution. 



