CHAPTER XIV 



DEVELOPMENT OF THE FERTILIZED EGG 



EMBRYOLOGY AND METAMORPHOSIS 



BY the term embryology is meant that part of the life his- 

 tory of the animal or plant which begins with the fertilization 

 of the egg and continues up to the time when a developed 

 animal is formed, ready to emerge from the egg as a free-living, 

 independent individual. When it hatches from the egg it is 

 sometimes like the adult, except in size; but sometimes it is 

 unlike its parents and goes through a further series of changes. 

 In this case we speak of these later stages as constituting the 

 larval history or a metamorphosis (Gr. meta = beyond + mor- 

 phe = form). The development of animals from the egg to 

 the adult stage, embryology and metamorphosis, has proved 

 to be an especially interesting phase of biological study, and 

 has received much attention in the last fifty years. The em- 

 bryology of different animals and plants differs widely, but 

 certain fundamental laws and rules are found to apply to all 

 alike. In this introductory study it is only possible to give 

 a few of the fundamental principles, using a single animal as 

 an illustration. For this purpose will be described the de- 

 velopment of the frog, which, although peculiar in some 

 respects, will illustrate the important laws both of embryology 

 and metamorphosis. The embryology of plants has also been 

 studied rather extensively, but has not hitherto yielded so 

 many interesting lessons as the embryology of animals. 



EMBRYOLOGY OF THE FROG 



1. Segmentation. The life of an individual frog may be 

 said to begin the instant that the nucleus of the egg fuses with 

 the head of the sperm (Fig. 121 H), the time of fertilization 

 being thus a starting point of a new life. This fertilization of 



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