The Germ-cells of Lophius. 575 



About this time the germ-cells migrate among the cells of the 

 splanchnic mesoblast, toward the medial border of the coelom, 

 and thence upwards into the somatic layer. Different stages of 

 this process are shown in figs. 7 and 8. Stage 11 marks the first 

 indication of the formation of the pronephric duct, which in stage 

 12 has become separated from the coelomic mesoblast through- 

 out the greater part of its length. About the same time begins 

 the growth of the coelom around the gut (figs. 8 and 9). In em- 

 bryos of stage 12 the germ-cells have reached the dorsal wall of 

 the coelom and are for the most part close to the pronephric 

 duct (fig. 9) . In one embryo of this stage a single germ-cell was 

 seen still in the splanchnic layer and near the side of the coelom 

 (fig. 9). This is the only aberrant germ-cell observed with cer- 

 tainty in any embryo. 



About stage 10 the blastopore closes (fig. 15), and at stage 12 

 the embryo is ready to break from the egg membrane (fig. 16). 

 Embryos of stage 12 are about 2.7 mm. in length and curve half 

 way around the egg. Stage 13 includes the newly hatched fish 

 with the yolk sac still large (fig. 17). Immediately after hatch- 

 ing the growth of the embryo is rapid, owing probably to the 

 absorption of water. The greater size is due largely to increase 

 in the amount of mesenchyme (cf. figs. 9 and 10). There also 

 begins at this time, a rapid increase in length. Fig. 17 shows an 

 embryo of this age measuring about 4 mm. 



It will be noticed that one of the very prominent changes at 

 this period is the very great and sudden enlargement of the coelom. 

 During this process the coelomic wall becomes very much thinner, 

 and this leaves the germ-cells standing out prominently on the 

 dorsal wall of the coelom, just below the pronephric duct. These 

 great changes in the embryo at the time of hatching cause 

 scarcely any change in the position and distribution of the germ- 

 cells, other than a noticeable scattering (fig. 10). 



By the time stage 14 is reached the yolk-sac is almost absorbed 

 and the fish has attained a length of 6 mm. (figs. 11 and 18). 

 The changes in internal structure have been great. The gut has 

 become larger and longer, forming one coil, and the coelom has 

 entirely surrounded it producing dorsal and ventral mesenteries. 



