216 BIOLOGY: GENERAL AND MEDICAL 



significance of this expression will be better understood 

 after the perusal of the matter that is to follow. 



Every metazoan begins its life history as a single cell 

 or egg, and whether this is a distinctly differentiated 

 germinal cell or egg or an indistinctly differentiated 

 germinal cell such as forms the starting point of the 

 gemmation of ccelenterates, etc., makes no essential 

 difference. 



For the present, however, we shall neglect the undif- 

 ferentiated and consider only the differentiated germinal 

 cells the true eggs. 



These present a great variety of appearances, but 

 little difference of structure, as each is a single cell. They 

 vary from a size so small as to be microscopic to several 

 pounds in weight (ostrich egg), may be naked and purely 

 protoplasmic or covered with membranous, leathery, or 

 calcareous encasements, these differences serving to 

 enable even a beginner to realize that there are phylo- 

 genetic differences even among the eggs themselves. 

 No doubt, increasing familiarity with eggs in general 

 will eventually show that the differences are not only 

 such as to enable eggs to be referred to their respective 

 phyla, but to their respective classes, orders, genera, 

 and even species, as can readily be done at present, for 

 example, with birds' eggs and many insects' eggs. 



Not only are there such external differences, but there 

 are also striking internal differences among eggs, which 

 not only assist in their classification, but also assist in 

 explaining peculiarities attending their development. 



Thus, a superficial examination enables one to separate 

 eggs into those that are holoblastic, or without yolks, 

 and those that are meroblastic and have yolks, and to 

 discover that though the eggs differ in size, as do the 

 other cells of the respective animals to which they belong, 

 the presence or absence of a yolk and the size of that 

 yolk have much to do with the size of the egg. The 

 yolk is, moreover, inclosed in the egg, which, according 

 to its size, is surrounded by a thicker or thinner proto- 

 plasmic envelope. The yolk which is composed of 



