GERM-CELL HISTORY IN THE BROOK LAMPREY 17 



depressions in the entoderm formed by the pressure of the 

 germ cells against it. The result is that in certain sections the 

 germ cells appear to be still in the entoderm. Such sections of 

 this late stage examined without knowledge of the previous his- 

 tory of the cells might lead one to believe that they were migrat- 

 ing from the entoderm into the mesoderm. But there is no reason 

 for believing that such a belated migration takes place in the 

 lamprey, for other sections show that practically all the meso- 

 derm has separated from the entoderm at this stage and that all 

 the germ cells which are destined to become functional now lie 

 in the mesoderm. Their position is about the same as in the 

 preceding stage. They have not yet reached the midline cranially, 

 while caudally they are scattered along the lateral plates of the 

 mesoderm so that at the very extreme caudal end they are still 

 found near the midventral line of the gut entoderm. 



In later stages germ cells which lie in the lateral plates, far 

 removed from their final destination, are often found in various 

 stages of disintegration. It is also likely that many prospective 

 germ cells never reach the mesoderm, but remain in the gut 

 entoderm either to degenerate in situ or to be thrown off. 



/. Larva 320 hours old. A larva of this stage is considerably 

 longer than that of the preceding stage. The caudal part of the 

 body is still slightly curved. Cranially the germ cells lie between 

 the. dorsal aorta and the pronephric ducts; caudally they lie ven- 

 trad or laterad of the ducts. The germ cells are in all respects 

 similar to those of the preceding stage. 



g. Larva 359^ hours old (fig. 9). The body of a larva of 

 this stage is almost straight (fig. 9) . The germ cells, two of which 

 are shown in figure 20 are nearer the middorsal line than before, 

 have lost their rounded contours, and are flattened between the 

 gut entoderm and the pronephric ducts, dorsal aorta, and inter- 

 vening mesenchyme. The germ cells are still filled with yolk 

 globules and there is no indication that mitosis is taking place. 

 The nucleus is usually eccentric and contains two deeply staining 

 nucleoli, besides scattered chromatin granules. Each germ cell 

 is surrounded by a number of flattened mesoderm cells. Figure 

 20 shows one germ cell cut through the nucleus and another cut 



