AN INTRODUCTION TO ZOOLOGY. 



primitive case of difierentiation into tissues. After the 

 earliest stages of growth, a third layer, the mesoderm, is 

 present,' lying between the two primary layers. 



It is only in comparatively simple forms of the Metazoa, or 

 in the very early stages of growth of the higher forms, that 

 these layers can be seen in their originaL continuity. In all 

 other cases, they have become greatly folded, contorted, 

 differentiated, and modified, so as to form the various organs 

 of the body. These layers, as they exist in the embryo, are 

 usually called Epiblast, Hypoblast, and Mesoblast, while 

 the names Ectoderm, Endoderm (or entoderm), and Mesoderm, 

 are usually applied in the case of adult animals ; the two sets 

 of terms are, howover, frequently interchanged (see fig. 23). 



6. Every animal in its earliest stage, the egg-cell, is 

 unicellular, like a Protozoan. This cell, after fertilization, 

 divides, and becomes by subsequent repeated divisions first 

 two-layered (diploblastic), and then ' three-layered (triplo- 

 blastio). By the further folding and modification of the 

 layers, the adult form is acquired. 



If the animal escapes from the egg before the adult form 

 is acquired, it is called a larva ; and if the development of 

 the larva proceeds by sudden and remarkable changes, it is 

 said to undergo m.etamorphosis. 



7. In the cycle of development which the young animal 

 undergoes, the character of the immediate parent is known to 

 be the last to be assumed, and the earlier stages are understood 

 to represent the characters of various ancestors, less and less 

 remote as development proceeds. (These characters appear 

 in forms modified by the circumstance of enclosure in the 

 egg, or by the conditions of a free larval existence, as the 

 case may be.) The history of the development of the young 

 animal, i.e. its ontogeny, thus epitomizes, in a modified 

 form, the history of the development of its kind, i.e. its 

 phylogeny ; and by comparison of various types -we obtain 

 evidence of the evolution of animal life, from simple uni- 



In all "but the lowest of the Metazoa. 



