THE HUMAN OVUM. 449 



from a stage corresponding to about a forty-eight hour chick 

 embryo, or to a nine-day rabbit embryo, the history of human 

 development has been determined in considerable detail. 



For this satisfactory condition of our knowledge we are very 

 largely indebted to the labours of Professor His, whose detailed 

 and careful descriptions, and splendid series of figures, form the 

 basis on which the account in the present chapter has been in 

 chief part founded. 



The total period of human development is usually estimated 

 at slightly under ten lunar months. The exact period cannot 

 be ascertained, owing to the impossibility of determining the 

 time at which fertilisation of the egg is effected, i.e. at which 

 development commences. 



The details of development of the human embryo are closely 

 similar to those of the rabbit ; the chief points of difference 

 being : (i) the far longer time occupied by the human embryo, 

 more than nine times as long as the rabbit ; (ii) the extreme 

 slowness with which the early stages of development are effected 

 in the human embryo ; and (iii) the early stage at which the 

 allantois is established in the human embryo, and the peculiar 

 mode in which it is formed. 



THE HUMAN OVUM. 

 1. Formation of the Ovum. 



The earlier stages in the development of the ova are already 

 completed in the female child before birth ; and after birth the 

 formation of ova only goes on for a very short time, and to a 

 very limited extent. According to Bischoff, Waldeyer, Foulis, 

 and others, the formation of new ova ceases about the age of 

 two years ; in other words, the ovaries of a female child already 

 contain, at the end of the second year, all the ova that will ever 

 be developed in them. As each ovum is morphologically a 

 single cell, this means that an individual cell may live, and 

 retain all its characteristic activities, for a period of forty-five 

 years or more. 



The early development of the human ova must, therefore, be 

 studied, not in the woman or child, but in the embryo. The 

 several stages of its formation are so closely similar to those' 



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