66 THE MAMMALIAN EGG 



generally seen as a small round body with a distinct core — the, 

 centriole. The centriole divides during the later phases of mitosis 

 and the centrosome soon afterwards, so that in the primary oocyte 

 there are first of all two centrioles within a single centrosome and 



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Fig. 54 

 Electron micrograph of an impenetrated golden-hamster egg, showing the cortical 

 granules. X 14,000. 



later two centrosomes each with a single centriole, and these 

 structures are disposed near the border of the nucleus. A similar 

 arrangement is found in embryos approaching the second and 

 subsequent cleavage divisions. At the start of the first meiotic 

 division, or of the second and subsequent cleavage mitoses, the 

 centrosomes take up positions at opposite poles of the nucleus, while 

 a characteristic radial or star-like structure, the aster, develops in the 

 cytoplasm immediately surrounding the centrosomes. When the 

 asters are fully grown, the nucleus appears to be supported between 

 them. With the condensation of the chromosomes and the dis- 

 appearance of the nuclear membrane, the achromatic spindle 

 develops between the asters and on this the chromosomes become 

 arranged. In the secondary oocyte, the centrosome at the vitelline 

 pole of the first maturation spindle divides to become the centres 

 of the asters and spindle of the second meiotic division. The cen- 

 trosome responsible for the origin of the first cleavage spindle has 

 been shown to arise in some species from the centriole introduced 



