340 Papers from the Department of Marine Biology. 



SUMMARY OF RESULTS. 



1. Twenty-five embryos of the loggerhead turtle (Caretta caretta), 

 ranging from the second day (5 somites, 2 mm. length) to the thirty- 

 second day of incubation, were employed in this investigation. The 

 early history of the primordial germ-cells is very similar to that de- 

 scribed by Allen for the turtle Chrysemys and by Woods for the dog- 

 fish. 



2. The primordial germ-cells migrate during the second day from the 

 yolk-sac entoderm, where they were more or less widely scattered 

 caudally, into the lateral border of the area pellucida on each side of 

 the embryonic disk. Here they become sharply segregated by the 

 beginning of the third day into two bilateral cords situated in the 

 entoderm of the area pellucida laterally in the caudal half of the disk. 

 In the 2-day embryo they extend from the neurenteric canal to the end 

 of the primitive streak; in the 3-day embryo from the sixth somite to 

 the caudal extremity of the streak. The cords become more medially 

 placed, make a linear connection with the overlying visceral mesoderm, 

 and their cells migrate during the fifth day into this mesoderm, and 

 thence medially (during the sixth and seventh days) towards the root 

 of the forming mesentery of the closing hind-gut. Individual cells 

 migrate medially also within, or back into, the entoderm of the gut. 

 The germ-cells in the medial entoderm become included in the mucosa 

 of the closed hind-gut, those in the mesoderm in the enveloping 

 mesenchyma and the gut end of the mesentery. From these locations 

 the majority of the germ-cells subsequently (seventh to twelfth day) 

 migrate up the mesentery and across the ccelomic angle into the future 

 sexual gland. They become incorporated among the mesenchymal 

 cells of the gland and the covering peritoneal epithelium, where they 

 suffer no striking change in form, size, or content at least as late as the 

 thirty-second day of incubation. 



3. The germ-cells migrate by amoeboid activity, assisted in small 

 part probably by the factor of unequal growth, involving the shifting 

 of the medial portion of the splanchnopleure to the mesentery and the 

 dorsal portion of the mesentery to the gonads. 



4. The migration period is not sharply limited. It is at its height 

 from the seventh to the twelfth day, and practically ceases about the 

 sixteenth day. But occasional extra-regional cells may still be found 

 in the gut and mesentery at the 32-day stage usually, however, showing 

 signs of degeneration. 



5. A certain number of germ-cells migrate out of the regular germ- 

 cell route and go astray. Such strays are especially numerous in the 

 periaortic mesenchyma caudally, where they may become incorporated 

 among the neuroblasts of the developing peripheral sympathetic 

 ganglia. The majority of these strays probably degenerate in situ, 

 but some may possibly persist to form, under the proper pathologic 



