EMBRYOLOGY 



207 



only in more complex aggregations, like Hydra. The occurrence of this 

 distinction between reproductive and non-reproductive cells in very sim- 

 ple aggregations of cells probably indicates that, in the evolution of 

 animal forms, germ cells arose at an early time. As if a reminder of the 

 historically early origin of germ cells in the evolution of the metazoa, 

 the reproductive cells appear at very early stages in the development of 

 individual animals today. Development has not proceeded very far 

 until, in most animals, the germ cells are easily recognizable. Further- 

 more, there is evidence to show that certain cells may actually be des- 

 tined to become germ cells from a time much earlier than their visible 

 differentiation. How early their fate is sealed is known in the case of 

 only very few animals; but it is not improbable that in most species germ 

 cells exist as definite germ cells, although not recognizable, almost from 

 the earliest stages of embryonic development. 



Origin of Germ Cells in the Invertebrates. — The early origin of germ 

 cells in the embryo has been demonstrated in Sagitta, the arrow-worm. 



Fig. 160. — Diagram illustrating the early recognition of the germ cells of the arrow- 

 worm Sagitta. At each of the early divisions the X-body (x) passes to only one cell; and 

 that cell gives rise eventually to all the definitive germ cells. {From original diagram by 

 P. O. Okkelberg.) 



In the egg of this animal a structure in the cytoplasm known as the x- 

 hody, is present while the egg is still unsegmented (Fig. 160). When the 

 egg divides, the x-body passes to only one of the cells. At the second 

 division it again passes undivided to one of the cells, the other three 

 lacking such a structure. In each of the next four divisions its behavior 

 is the same, so that of the 64 cells present at the end of that time only one 

 contains the x-body. In the seventh division, however, this body divides 

 and passes to the two daughter cells. From these two cells, out of a total 

 of 128, come the germ cells of the worm that is to develop from the em- 

 bryo. The question whether the x-body causes the cells that contain it 

 to become germ cells need not be debated; but that it distinguishes them 

 as germ cells is certain. 



In Ascaris megalocephala, a roundworm parasitic in the intestine of 

 the horse, the germ cells are early recognizable from the behavior of their 

 chromosomes in cell division. When the first two cells derived from. the 

 fertilized egg divide to form four cells, the chromosomes of one cell re- 

 main intact, while in the other the thickened ends of the chromosomes 



