FERTILISATION 125 



radiate structure known as the sperm-aster. The head of the 

 spermatozoon is transformed into an elongated nucleus lying 

 to the outer side of the sperm-aster, and the two move together 

 towards the interior of the egg. The path along which the 

 sperm nucleus and aster move determines the plane of the 

 first division of the fertilised egg. "While the sperm-nucleus 

 is travelling inwards the egg-nucleus leaves the position it has 

 hitherto occupied at the pigmented pole of the ovum, and also 

 travels inwards to meet the sperm-nucleus at the point which 

 may be described as lying in the axis of the egg at about a 

 quarter of the length of its diameter from the pigmented pole. 

 The centrosome of the spermatozoon disappeared along with 

 the middle piece, but now as the sperm- and egg-nuclei approach 

 a new centrosome makes its appearance in the sperm-aster. 

 This centrosome occupies a position between the sperm- 

 nucleus and egg-nucleus, and as they approach one another 

 it divides in a direction at right angles to the path along 

 which the sperm-nucleus has travelled. The two centrosomes 

 thus formed diverge from one another and the radiations of 

 the sperm-aster are collected round them to form the polar 

 radiations of the future mitotic spindle. Meanwhile the 

 sperm-and-egg nuclei have increased in size and exhibit 

 changes similar to those of nuclei about to entei upon 

 division. Spindle fibres are formed in connection with the 

 centrosomes and are attached to the two nuclei. Twelve 

 chromosomes now emerge from the reticulum of each nucleus 

 and are attached to the spindle fibres in two distinct and 

 separate groups. Each chromosome is split into halves 

 longitudinally, and the halves are pulled apart and carried 

 to the two poles of the spindle by the fibres. An equatorial 

 plate is formed in the spindle, and the whole ovum divides 

 into two, the segmentation furrow passing through the equator 

 of the spindle and therefore coinciding with the path of 

 entrance of the spermatozoon into the egg. It is noteworthy 

 that this process not only provides for the restoration of the 

 full number of chromosomes but also ensures that each of the 

 two segments or blastomeres into which the egg is divided 

 shall have half of its chromosomes derived from the male 

 and half from the female parent. The importance of this fact 

 in relation to the transmission of hereditary qualities to the 

 offspring cannot be overlooked. 



