16 The Microscope. 



one does not have to make those pilgrimages to the sea-shore 

 that are necessary to study the development of so many types 

 of animal life. So, with the view of interesting inland students 

 in this department of study by placing before them a short ab- 

 stract of what is at present known and the points upon which 

 information is desired, together with suggestions upon the 

 methods of study, the present series is prepared for The Micro- 

 scope. 



In order to make the account intelligible, we must preface 

 it by a general account of the early stages of development of 

 all ot the higher groups of the animal kingdom, taking up first 

 the typical processes and then the variations, showing at the 

 same time their similarities and the causes of the dillerences 

 observed in nature. After this the peculiar modifications found 

 in the insects will be more easily understood. 



Naturalists have divided the animal kingdom into two great 

 groups, the Protozoa and the Metazoa. In the former the ani- 

 mal consists of but a single cell, in the latter of an aggregation 

 of cells, variously combined. Each protozoon possesses in itself 

 all the vital functions of the higher animals, though of course 

 in a very low condition. Thus the one cell of these animals 

 possesses the powers of sensation, motion, digestion, circulation, 

 excretion, reproduction, etc. In the Metozoa we find a differ- 

 entiation of functions. Accordingly as the animal is simple or 

 complex, this specialization is carried farther and farther. In- 

 stead of each cell having all the functions of the single cell of 

 the protozoon, the cells are united into tissues, one of which, 

 for instance, has sensory, another digestive, and a third motor 

 functions. Now in all the Metazoa these various tissues have 

 all arisen from a single cell, morphologically equivalent to the 

 single cell of the Protozoa, and this process of the differentia- 

 tion of functions and of tissues is that studied by embryologists. 

 The differences between a complex animal like man and a 

 single celled animal seem immense, but the various steps can 

 be traced with unerring accuracy, and it is found that the transi- 

 tions are not sudden, but gradual. 



The reproduction of a protozoon is always accomplished by 

 division. At certain periods one of these complete animals will 

 divide, and the result is two or more individuals like the first.* 



* Reproduction by encystment is but a modified form of the more common 

 reproduction by division, and does not need mention here. 



