330 Papers from the Department of Marine Biology. 



from the entoderm near the lateral margin of the area pellucida at a 

 level beginning about the cephalic extremity of the pronephros and 

 extending on either side to a point just behind the caudal tip of the 

 embryo. The total number of the original quota of germ-cells is said 

 to vary from 302 to 1,744, with an average of about 1,100 (Allen, 1907). 

 No mitoses were observed in these cells until the 10-mm. stage of 

 development. During the migration of the sex-cells many are said to 

 lag hopelessly behind, while others may go entirely astray. Upon an 

 average 47.7 per cent are estimated to finally reach the sex-glands, 

 while the remainder are said to come to rest in the alimentary tract, the 

 mesentery, and the region between the root of the mesentery, the aorta, 

 and the mesonephroi. 



It is important to note that Dustin (1910) records a much smaller 

 average number of germ-cells in Chrysemys embryos of this stage, the 

 extremes being given as 158 and 415. This number is much closer to 

 my own count (see chart, page 323) for Careita. 



In Lepidosteus the primordial germ-cells have a very similar origin. 

 They are described as migratory to a high degree. In Amia they are 

 said to arise from the entoderm of the roof and margin of the floor of 

 the subgerminal cavity. 



The history of the germ-cells in Chrysemys is practically identical 

 with that above described for Caretta. However, I find no evidence in 

 Caretta to support Allen's conclusion that ''large numbers" of cells 

 fail to migrate eventually to the gonads, but undergo degeneration in 

 these extra-regional locations. Rarity of actual degenerating cells, or 

 persisting representatives of any kind in later stages, indicates an 

 ultimate migration of at least the vast majority of the extra-regional 

 germ-cells into the genital-glands. Moreover, an occasional germ-cell 

 may be seen dividing during all the earlier stages. 



Dustin (1907) describes the origin of the sex-cells ("gonocytes") 

 in a 3 mm. Triton larva from the medial portions of the lateral plates of 

 mesoderm in the caudal half of the body. These original sex-cell 

 masses become approximated medially and subsequently fuse to form a 

 longitudinal rod of sex-cells just above the dorsal root of the mesentery. 

 At about the 14 mm. stage the majority of the original sex-cells are 

 said to degenerate and disappear, and a new generation to be differen- 

 tiated from the peritoneal epithelium of the genital ridge. A similar 

 mode of origin and history of the germ-cells is described for Rana and 

 Bufo. As regards Rana pipiens, Allen (1907) describes a dorso-medial 

 migration of germ-cells from the gut entoderm at the time when the 

 two lateral plates are approximated in the formation of the mesentery. 

 The initial mesodermal origin described by Dustin is accordingly here 

 an illusion. 



In the matter of an entodermal origin of the germ-cells in Rana, 

 Allen has been confirmed by Kuschakewitsch (1908), and by King 



