14 PETER OKKELBERG 



on either side of the nerve cord and notochord, and these are 

 bounded externally by the ectoderm. The cells of the mesento- 

 derm vary in size, are irregular in shape, and all of them are 

 heavily laden with yolk. Several larger rounded cells stand out 

 rather clearly among the more irregular surrounding cells. One 

 of the most cranial of these is shown in figure 11. It lies directly 

 under the ectoderm, in that part of the mesentoderm which later 

 forms the mesoderm. Many other cells similar to this one are 

 found farther caudad. That these are germ cells is shown by 

 their later history, and the posterior region of the mesentoderm 

 may therefore be considered a region for the proliferation of these 

 cells. The shaded area in figure 5 shows in a diagrammatic 

 way the relative position of the germ cells at this stage. 



b. Embryo 238 hours old (fig. 6). A sketch of an embryo 238 

 hours old, about one day before hatching, is shown in figure 6, 

 and a section along the line ab is shown in figure 12 In the 

 latter figure three spherical germ cells, structurally identical 

 with those of the preceding stage, are seen in the lateral portions 

 of the mesoderm which have recently separated from the ento- 

 derm. Other germ cells may be recognized caudad of this sec- 

 tion and also considerably farther craniad. The relative posi- 

 tion of the cells in the whole embryo is shown by the shaded 

 area in figure 6, and it will be seen that they extend much farther 

 craniad than in the preceding stage. They are not arranged in 

 regular groups in relation to the body somites, but form irregular 

 bands, one on each side of the body. At this stage the dorsal 

 part of the mesoderm has separated from the entoderm along its 

 whole length, but its lateral plates are still continuous ventrally 

 with the entoderm except along the cranial part of the germ-cell 

 region, and craniad of it where a complete separation seems to 

 have taken place, but even here one cannot be absolutely certain 

 that the two germ layers are not continuous ventrally. Each 

 lateral plate becomes thinner along the side of the body and 

 terminates ventrally in a sharp edge, but the separation 

 of this edge from the entoderm is not distinct. This makes it 

 very difficult to determine whether the mesoderm extends 

 ventrad at the expense of the entoderm or by independent 

 growth. 



