GERM-CELL HISTORY IN THE BROOK LAMPREY 43 



and follicle cells by their smaller size, structure, and rounded 

 form. The cytoplasm ramifies in all directions so as to form a 

 loose parenchymatous structure (fig. 36). Blood-vessels enter 

 the gland from above Avith the mesenchyme and fill practically 

 the whole of some sections. The endothelium of these blood- 

 vessels consists of the usual flattened cells containing much 

 flattened nuclei. 



The germ cells remain as distinct elements with more or less 

 clear cytoplasm, usually without observable limiting mem- 

 branes, and with large rounded nuclei. In each nucleus there 

 are, as before, two deeply staining nucleoli and a distinct chro- 

 matin network. One has no difficulty in distinguishing the 

 germ cells from the somatic elements of the gland by their 

 nuclear structure and the much greater size of both nuclei and 

 cell bodies. No transitional stages were found between the 

 germ cells and the somatic cells of the gland, and no indication 

 of the transformation of somatic elements into germ cells or germ 

 cells into somatic cells. The number of the germ cells is in- 

 creased rapidly during this stage, but so many of them are in 

 mitosis that the increase in their number may easily be accounted 

 for without supposing that new germ cells are being formed out 

 of surrounding tissue cells. 



h. Larva 25 mm. long (fig. 37). The structure of the germ 

 gland in a larva of this stage is similar to that of the preceding 

 stage, except that, owing to the enlargement of the gland without 

 corresponding increase in the number of germ cells and cysts, 

 these are more scattered than in the earlier stages. In many sec- 

 tions not a single germ cell occurs. In some sections both germ 

 cells and blood-vessels are absent and the peritoneum encloses 

 only a loose stroma with greatly scattered nuclei. 



The gland is now suspended from the middorsal line of the 

 coelom by a broad mesentery, in which germ cells are present, 

 while some are still found even in the mesenchyme above it. The 

 germ cells and cysts often lie against the peritoneum and new 

 folHcle cells are being formed continually by division and migra- 

 tion of peritoneal cells. 



