THE REPRODUCTION OF ANIMALS 231 



complex, many-celled organism more or less like the parent. It will be 

 impossible for us to follow this development in detail for even one kind 

 of organism, but we can see something of embryonic processes in general 

 and learn to recognize embryonic stages that are common to all the 

 metazoa. 



Every zygote contains a diploid nucleus, a small mass of cytoplasm, 

 and more or less stored food material in the form of yolk. The nucleus, 

 as we have seen, was formed by the fusion of the haploid egg and sperm 

 nuclei, but the cytoplasm and yolk were contributed solely by the egg, 

 in which the proportions of cytoplasm and yolk and the details of their 

 arrangement had been determined before fertilization. The proportions 

 and organization of the cytoplasm and yolk are characteristic of the 

 general group to which the animal belongs and will have two important 

 consequences in the development of the zygote. Since in all oviparous 

 eggs the yolk provides the chief or sole material and energy for growth, 

 the amount of yolk will determine how far the purely embryonic develop- 

 ment can go before the young individual will be thrown upon its own 

 resources or before the parents will need to provide another food supply. 1 

 On the other hand, since the yolk is nonliving and inert, it cannot take 

 any active part in development, and, if present in a considerable amount, 

 it modifies or, so to speak, distorts the processes that are carried on by 

 the living cytopla.sm. 



There is good reason to believe that the homolecithal egg, described 

 below, shows the most primitive organization of cytoplasm and yolk. 

 The other types of eggs are apparently modifications of this original 

 organization caused by the inclusion of large stores of yolk, which, al- 

 though they variously distort its early stages, provide for a more ade- 

 quate embryonic development. On this hypothesis it is possible to 

 correlate the many fundamental similarities that are common to the 

 early embryonic development of all metazoans (except the sponges) and 

 to interpret the relatively minor differences between them. 



It should be noted that all eggs, and hence the zygotes derived from 

 them, show polarity; i.e., they are so organized that a certain point on 

 the surface, the animal pole 2 is destined to be the center of the early 

 externally visible embryonic activity. The opposite vegetative or vegetal 

 pole marks the center of the region of greatest concentration of yolk and 

 hence of least early activity. 



1 Parental care in the birds, certain of the insects, and a few other forms that have 

 already provided a large store of food within the egg still further postpones the neces- 

 sity of the young being "on their own" at the completion of embryonic development. 

 The young of many of the Metazoa, however, must begin their independent existence 

 long before development has reached anything like the adult structure of the parents. 



2 The egg nucleus is nearer this point than to any other part of the surface, and 

 the extruded "polar bodies" are usually found here. 



