198 BIOLOGY: GENERAL AND MEDICAL 



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 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 

 deuteroplasm is intended to nourish the developing 

 embryo, hence the magnitude of the yolk must bear 

 some reference to the dependence of the embryo upon 

 that form of nourishment. In cases in which the egg is 

 quickly developed into a self-sustaining larva, there is 

 no yolk; in cases where it becomes attached to the 

 uterine wall of the parent from whom it derives nourish- 

 ment, the yolk may be inconspicuous, but in those cases 

 the birds, reptiles, fishes where the egg is entirely 

 separated from the parent and completes its embryonal 

 transformations without a larval form in which addi- 

 tional nourishment can be secured from without, the 



