VERTEBRATE EMBRYOLOGY, 



CHAPTER I. 

 INTRODUCTION. 



General Account of the Development of Animals. 



ALL animals may be referred to one or other of two great groups. 

 Protozoa and Metazoa. Of these, the former, or Protozoa, are 

 minute, often microscopic animals, which throughout their whole 

 lives remain single cells. Most Protozoa lead solitary existences, 

 but there are several that give rise to colonies by continuous 

 fission ; in such colonies, however, each member, though organi- 

 cally connected with its neighbours, is physiologically indepen- 

 dent of them, and discharges all the great functions of life for 

 itself. 



The second group, or Metazoa, includes all remaining animals, 

 from sponges to man. It is characterised by the fact that the 

 adult animal consists, not of a single cell, but of many cells, 

 which are variously modified in different parts to form the diges- 

 tive, respiratory, nervous, and other organs. 



In a Protozoon, such as Amoeba or Paramecium, the entire 

 animal is a single unit or cell, and all the activities of the 

 living organism nutrition, respiration, sensation, &c. have to 

 be carried on within the compass of that single cell. 



A Metazoon, on the other hand, such as a jelly-fish, a snail, a 

 beetle, or a frog, is built up of a number of such units or cells, 

 which share the work of life amongst themselves. 



In the simpler Metazoa, as sponges, or zoophytes, there are 

 comparatively few kinds of cells, an outer protective and sensory 

 layer, and an inner digestive layer being the most conspicuous. 



B 



