Embryonic History of the Germ-Cells of the Loggerhead Turtle. 339 



thelial cells and the definitive germ-cells more close. At any rate, no 

 clear case has yet been made out for the differentiation of germ-cells 

 from ccelomic epithelium at any stage in any form. 



There is no evidence in Caretta of a differential mitochondrial 

 content between the germ-cells and somatic cells, as maintained by 

 Tschaschin in the case of the chick embryo and by Rubaschkin in the 

 case of certain mammalian embryos; and as denied by Swift and von 

 Berenberg-Gossler. The germ-cells are distinguished from the so- 

 matic cells by their larger size, their generous yolk-content, their large 

 vesicular oxyphilic nucleus, with its delicate, finely granular, radially 

 arranged, nuclear network. These nuclei resemble more closely 

 those of the entoderm-cells of the area opaca, indicating a low grade 

 of differentiation of these two cells and a close genetic relationship 

 inhering most probably in a common near ancestor, from which they 

 have departed but little in their slight progressive differentiation. This 

 indicates also that the germ-cell commonly remains undifferentiated 

 until a relatively late period in its history, when it takes residence in 

 the genital gland. Its load of nutritive material in the form of yolk 

 (still present in the 25-day stage) also indicates a low grade of differen- 

 tiation and suggests the cause of its lack of proliferative capacity until 

 digestion of this yolk is completed, and the further reason of its great 

 capacity for later proliferation and differentiation, especially in the 

 male. 



But many more researches covering the later periods of the history 

 of the ovary with special reference to the so-called " germinal epithe- 

 lium," such as those of Swift on the chick and of Gatenby on the frog, 

 are needed before the hypothesis of a germinal path for vertebrates can 

 be said to be completely established. In view of conditions in the 

 gonads of certain annelids, mollusks, and echinoderms, where enormous 

 numbers of germ-cells are formed during successive years, and the 

 disagreements which still exist with respect to such relatively simple 

 forms as frog (Allen and King vs. Dustin and Gatenby), chick (Swift 

 vs. Firket), and the turtle, Chrysemys (Allen vs. Dustin), the claims 

 that two series of genital cells occur a primordial and a secondary or 

 definitive the one derived directly from a blastomere of the later 

 cleavage stages and so segregated from the somatic cells, and the 

 other derived by differentiation from the ccelomic epithelium of the 

 genital ridge, and so modified soma-cells, while on logical grounds 

 inherently improbable, and without firm observational basis and 

 disproved on histologic grounds, I believe, in the case of Caretta can 

 not perhaps be said to have been definitely disposed of. 



