CHAPTER X. 
DEVELOPMENT AND METAMORPHOSES OF ANTI. 
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 
