CHAPTEE LIX 



DEVELOPMENT 



THE description of the origin and formation of the tissues and organs 

 constitutes the portion of biological science known as embryology. 

 All one can possibly attempt in a physiological text-book is the 

 merest outline of the principal facts of development. 



In our descriptions we shall speak principally of the develop- 

 ment of the mammal ; it will not be possible to do so altogether, 

 for many of the facts which are believed to be true of the mammal 

 (man included) have only been actually seen in the lower animals. 

 That they occur in the higher animals is a matter of inference. 



It will, however, add interest to the subject to draw some of our 

 descriptions from the lower animals ; for the scientific discussion of 

 embryology must always start from a wide survey of the whole 

 animal kingdom, because the changes which occur in the embryo- 

 logical history of the highest animals, form a compressed picture 

 of the changes which have taken place in their historical develop- 

 ment from lower types. 



The Ovum. 



The human ovum is like that of other mammals, a small cell 

 about T | T to yi^ inch in diameter. 



The changes by which the ovum, or a portion of the ovum, is 

 transformed into the young animal may take place either inside or 

 outside the body of the parent. If they take place inside the parent, 

 as in mammals, including the human subject, the ovum is small, and 

 the nutriment necessary for its growth and development is derived 

 from the surrounding tissues and fluids of the mother. If the 

 development takes place outside the parent's body, as in birds, the 

 egg is larger ; it contains a large amount of nutritive material 

 called the yolk, and it may, in addition, be surrounded by sheaths of 

 nutritive substance. Thus, in the hen's egg, the yellow part alone is 

 comparable with the mammalian ovum, and the larger part of that 

 is merely nutritive substance; upon it is a whitish speck, the 



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