22 PETER OKKELBERG 



The germ cells lie against the peritoneum which covers the 

 gland (fig. 30), but they are always separated from the coelom 

 by peritoneal cells and they have never been found to form a 

 part of the peritoneal epithelium. Epithelial cells may be 

 seen, still in part included in the peritoneum, but with processes 

 extending to the germ cells and forming a part of their follicles. 

 These cells occur in all stages of detachment from the peritoneal 

 epithelium and in all stages of inclusion in the follicular mem- 

 branes of the germ cells. There is no evidence at this stage that 

 the follicular cells are derived from any other source. Both in 

 the peritoneum and in the follicles these cells are distinguishable 

 from the germ cells and mesenchyme cells by their ovoid nuclei, 

 each with one large plasmosome and several smaller chromatin 

 nucleoli, and by their flattened form and indefinite contours. The 

 mesenchyme cells are recognizable by their nearly spherical 

 nuclei, and the germ cells by their large size and spherical nuclei, 

 each with two large plasmosomes. It seems clear from their 

 early history, from the fact that they are at no time seen to be 

 included in the peritoneal epithelium, and from their distinguish- 

 ing structural characters, that the germ cells are not derivatives 

 of the peritoneal epithelium. It seems equally clear that the 

 follicle cells are derived from this source. 



2. Historical and critical, a. Invertebrates. A study of the 

 early history of the germ cells in various species of invertebrates 

 has disclosed the fact that they often are segregated during early 

 cleavage. These primordial germ cells are at first distinguished 

 either by the behavior of their chromosomes or by the presence 

 of certain cytoplasmic inclusions. In vertebrates the primordial 

 germ cells are usually not recognizable until the three germ layers 

 are formed, although most investigators of the subject (Beard, 

 Allen, Dodds, Nussbaum, King, Witschi, Rubaschkin, Swift, 

 Tschaschkin, and others) believe they must have been segre- 

 gated at a much earlier stage. The stanchest adherents of the 

 theory of early segregation (Beard, Allen, Rubaschkin, Witschi 

 Swift, and others) hold that all the definitive germ cells are 

 derived from the primordial germ cells, a conclusion that the 

 theory of the continuity of germ plasm naturally demands. 



