GERM-CELL HISTORY IN THE BROOK LAMPREY 19 



k. Larva 538^ hours (5tV mm. long) (fig. 26). In the caudal 

 region of the body the germ cells still retain their embryonic form. 

 They are apparently not yet able to reach their final median posi- 

 tion on account of the large amount of yolk in the entoderm of 

 this region. Cranially, on the other hand, the yolk in the ento- 

 derm is being absorbed so that more space is left for the germ 

 cells, and in consequence they shift their position toward the 

 midline. As a result, the two bands of germ cells are now 

 arranged in the form of a V with the apex pointing craniad. 

 With the release of pressure and with the assumption of a median 

 position, the anterior cells begin to show signs of activity. The 

 yolk globules in many cells have lost their sharp contours and 

 often appear fragmented (fig. 25). Sometimes they are absent 

 from certain parts of the cells. The cytoplasm, hitherto clear, 

 now has a granular appearance. The chromatin material in the 

 nucleus now stains more deeply, and often chromatin-like gran- 

 ules are found in the cytoplasm surrounding the nucleus. It 

 appears probable that there is at this time an active inter- 

 exchange of material between the nucleus and the cytoplasm. No 

 mitoses were observed, however, for a long time subsequent to 

 this stage. 



/. Larva 647 hours. At this stage the coelomic cavities have 

 formed by a splitting of the lateral plates throughout their length, 

 and the cavities of the two sides have fused and nearly the whole 

 of the dorsal mesentery has disappeared. The germ cells are 

 included in the somatic layer of the mesoderm. Most of them 

 now lie along the dorsal midline directly below the dorsal aorta, 

 but are spread out over a considerable area on each side of it. 

 All have disappeared from the caudal region. No germinal 

 fold is yet present. Some of the germ cells have lost the greater 

 part of their yolk. None of the cells were found in mitosis. 



7n. Larva 902^ hours. A few germ cells along the posterior 

 portion of the body cavity still retain some yolk, but the great 

 majority of them are now free from it. As compared with the 

 mesodermal cells of the same region, they may be described as 

 large spherical cells with large spherical nuclei each with two 

 large nucleoli. No cells were found in mitosis. Sometimes 



