16 PETER OKKELBERG 



its separation from the entoderm extends caudad. There is 

 no indication at this or at any other stage that the germ cells 

 are segmentally arranged. They form two bands which are sepa- 

 rated caudally, but converge cranially. The most cranial cells, 

 although older in the sense that they were first to be included in 

 the mesoderm as it became separated from the entoderm, are 

 apparently not different from the posterior cells which were 

 included much later. 



d. Larva 286| hours. This stage, shown in section in figure 18, 

 is somewhat more advanced than the preceding. The mesoder- 

 mic somites in the anterior region have become differentiated 

 into a muscle plate, a dermal plate, and a sclerotome. The 

 pronephric ducts lie in the regions laterad of the muscle plates. 

 Between them and the yolk entoderm, and sometimes indenting 

 the latter, are the large yolk-laden germ cells. They are of the 

 same size and structure as the cells of the preceding stage. 

 Occasionally the cells are found in groups, but no mitosis has 

 ever been observed in the germ cells of this period, and this 

 makes it probable that the cell aggregations are the result of a 

 slight amount of migration or of several cells being separated 

 from the entoderm at the same place. Due to a pressure from 

 surrounding cells, many of the germ cells have lost their rounded 

 appearance at this stage. The bands of germ cells of the two sides 

 approach each other more closely cranially than in the preceding 

 stage, but caudally they still lie far apart. 



e. Larva 299^ hours (fig; 8). The larvae of this stage have 

 increased considerably in length, but the caudal region is still 

 loaded with yolk and remains perpendicular to the rest of the 

 body (fig. 8). Figure 17 represents a section through the poste- 

 rior part of the body, from the region indicated by the line ah in 

 figure 8. In this section four germ cells are scattered along the 

 lateral plates of the mesoderm. A section near the cranial end 

 of the germ-cell area is shown in figure 19. This contains three 

 germ cells which now have reached a position mediad of the 

 pronephric ducts. Not only do the entoderm and mesoderm lie 

 so close against each other that it is difficult in some places to 

 see the line of separation, but the germ cells often lie in little 



