APPENDIX A. 
157 
of its predecessors in evolution, and its directed differentiation 
toward its future functioning. 
I. GERM CELLS 
For the beginning of an embryo, one can look both at the newly 
fertilized egg, and also further back, to embryos of the previous 
generation. The beginning of an individual is, of course, the union of 
egg and sperm, specifically the union of DNA in the nucleus of each, 
so as to form a new complete genome. But the egg and sperm in turn 
develop from primordial germ cells that were themselves developed 
when the parents of the new individual were embryos. This 
description starts at that point (Figure 1). 
i 
I 
1 
Figure 2.1 \ 
Dtfvciopmental history of a frog. The stage* from fmiliialion 
through haahing Chinh) are known collectively as embrvogenesU. 
Hatching 
(birth ) 
Figure 1: Developmental cycle, here of a frog. Note the continuity of germ plasm. 
[Figure 2.1, page 26, in Gilbert, S. Developmental Biology. 6*** Edition. Sunderland, 
Mass.: Sinauer Associates Inc., 2000. Figure reproduced with permission of Sinauer 
Associates.] 
The primordial germ cells are the cells that will give rise to either 
ova or sperm. They are large cells with some distinctive 
characteristics that make it possible to track them in development. 
Note that in Figure 1, they are highlighted throughout the life cycle of 
the animal. Primordial germ cells appear in embryonic development 
prior to the formation of the gonads (ovaries in female, or testes in a 
male). In humans and other mammals, the primordial germ cells 
actually develop first in the yolk sac. In either sex, the primordial 
germ cells migrate in through the developing gut of the embryo and 
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