BIRDS' EGGS 



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the mother must in this respect count for more than the 

 father. The ovum furnishes more of the initial building 

 material — " the organ-forming substances." When there 

 is, as in mammals, a close ante-natal partnership between 

 mother and offspring, there is another way in which the 

 mother counts for more than the father. But there is 

 nothing of this sort among birds, except in so far as it is the 

 mother that furnishes all the legacy of yolk on which the 

 developing embryo depends until it is hatched. 



(b) Fertilisation implies a restoration of the number of 



chromosomes to the normal ( - 4-- = « ), and in the fact 



that each parent furnishes only half the normal number of 

 chromosomes, there is a hint of the important conclusion 

 that the male and female parents contribute different items — 

 more subtly than is suggested by warp and woof — to the 

 web that will be woven, namely, the young creature. 



(c) The spermatozoon introduces into the egg-cell a 

 minute body called the centrosome, which plays an im- 

 portant part in the segmentation or cleavage that is shortly 

 to follow. This centrosome divides into two and these take 

 up their positions at opposite poles of the conjoint nucleus 

 and give origin to delicate plasmic radiations which have 

 something to do with the division of the chromosomes to 

 form two daughter-nuclei in the first segmentation or 

 cleavage. In every subsequent cell-division, all through 

 development and after, two centrosomes play the part of 

 " the weavers at the loom." They are centres of great 

 protoplasmic activity. 



(d) Whenever a spermatozoon has entered an ovum, a 

 rapid change occurs in the periphery, which is partly of the 

 nature of an oxidation. The result is a " blocking " of the 

 egg to the entrance of other spermatozoa. The egg-cell 

 becomes, as it is said, *' non-receptive." Although there 

 are cases where several spermatozoa enter an egg-cell at once 

 without doing apparent harm, there are other cases known 

 where " polyspermy," as it is called, leads to monstrosities 

 by setting up several non-unified centres of cleavage. 



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