GERM-CELL HISTORY IN THE BROOK LAMPREY 15 



In the caudal region of the body the germ cells are large and 

 spherical and stand out clearly among the apparently smaller 

 and more irregular-shaped cells of the yolk entoderm, as shown 

 in figure 13, a section from the region cd of figure 6. Farther 

 craniad the germ cells are more irregular in shape due to pres- 

 sure from surrounding cells. No structural difference could be 

 found between the germ cells and the large yolk-bearing cells of 

 the entoderm, except that the former have more definite out- 

 lines. It is probable that even this difference is the result of 

 location rather than of any inherent difference in structure. 

 This suggests the possibility that any of the yolk-bearing cells 

 of the mesentoderm which are so situated that they have a 

 chance to get into the mesoderm at the time it separates from 

 the entoderm may become germ cells. Another probability is 

 that the germ cells are segregated in an earlier stage and that 

 many more are produced in the early development of the embryo 

 than can be included in the mesoderm when it separates. In 

 this case all of the germ cells which remain in the entoderm prob- 

 ably degenerate in situ or are thrown off bodily into the lumen 

 of the intestine. Later some evidence for this will be presented. 



c. Larva 274 hours old (fig. 7). The larva of this stage has 

 just broken out of the egg membrane and the anterior portion 

 of the body has straightened out, as shown in figure 7. The 

 caudal region, however, which includes most of the yolk, still 

 forms a right angle with the cranial region. The position of the 

 germ cells from three different regions is shown in figures 14, 

 15, and 16, taken from the parts of the larvae indicated by the 

 lines ah, cd, and ef, respectively. At this stage the mesoderm 

 extends farther ventrad than in the preceding stage. The germ 

 cells lie in the nephrotome region, either ventrad or latero- 

 ventrad of the newly formed pronephric ducts. They are 

 much more numerous than in the preceding stage, and some- 

 times they lie so close together that in every section two or 

 more cells are found. The absence of mitotic figures and the 

 uniform size of the germ cells indicate that the increase in num- 

 ber is not due to any division of the cells, but to the fact that 

 more and more germ cells are being included in the mesoderm as 



JOURNAL OP MORPHOLOGY, VOL. 35, NO. 1 



