CHAPTER X. 



DEVELOPMENT AND METAMORPHOSES OP ANI- 



MALS, 



Embryology.— The development of the individual is often 

 an epitome of the classification of the order or class to which 

 it belongs, as well as of the development or appearance in 

 geological history of the different members of the order or 

 class to which the individual belongs. The changes under- 

 gone by the animal within the egg are often so sudden and 

 marked that the separate chapters of its history as an em- 

 bryo can be read side by side with the history of the succes- 

 sion of the different genera and families of its type in past 

 ages. Moreover, it is now generally supposed by naturalists 

 that these critical periods in the development of the individ- 

 ual have a constant relation to external causes which have 

 acted on the ancestors of the animal, and hence that these 

 changes are the result of influences and changes in the sur- 

 roundings of the forms which have preceded. So much in- 

 terest, therefore, attaches to the subject of the early develop- 

 ment of animals, that much prominence has in the foregoing 

 pages been given to the matter. 



We may now briefly review the more striking phenomena 

 of development in the invertebrate animals, and close with a 

 summary of the mode of development of Vertebrates. 



The eggs of all animals consist of three portions, the egg 

 proper, consisting of a mass of protoplasm enveloped by 

 the yolk or food-stuff, the nucleus or germinative vesicle, 

 and the nucleolus or germinative spot. 



Before the egg is ready for fertilization it undergoes a 

 special process of maturation, involving the following series 



