328 Papers from the Department of Marine Biology. 



of the germ-cells at the 8-cell stage, in a number of other cases at the 

 32-cell stage. 



The literature touching the germinal path ("Keimbahn") in inverte- 

 brates is fully and very ably reviewed by Hegner in the Journal of 

 Morphology (vol. 20, p. 231 ; vol. 25, p. 375; and vol. 26, p. 495). The 

 reader is referred to these papers for bibliographic lists. 



GERM-CELL ORIGIN IN VERTEBRATES. 



The detailed and critical study of the origin of the germ-cells in 

 vertebrates may be said to have begun with the observations of 

 Hoffman (1893) on certain birds (including Gattinula chloropus, Sterna 

 paradisea, and Hcemotopus ostralegus), which disclosed primordial germ- 

 cells in the entoderm and splanchnic mesoderm far removed from the 

 site of the future gonads, and before a " germinal epithelium" had 

 developed (23-somite stage). 



The work of Eigenmann (1892, 1897) marked an epoch in this line 

 of investigation. He clearly demonstrated the extra-regional origin 

 of primordial germ-cells in the viviparous teleost Cymatogaster and 

 showed that they closely approximated the size and appearance of the 

 fifth-cleavage blastomeres, on the basis of which he argued that the 

 germ-cells were segregated at the fifth-cleavage stage. An extra- 

 regional entodermal origin of primordial germ-cells has been described 

 also by Dodds (1910) in the teleost Lophius. 



In 1904 D'Hollander described the origin of the germ-cells (oogonia) 

 in the chick embryo of the tenth day from epithelial buds of the peri- 

 toneal epithelium, in conformity with the earlier views of Waldeyer. 

 This discrepant observation is now elucidated by the recent works of 

 Swift (1915, 1916), which show that such buds do indeed occur, but 

 that they include, but do not originate, primordial germ-cells. The 

 latter have an extra -regional origin. 



Beard in 1904 described the extra-regional origin of primordial germ- 

 cells also in the skate Raja batis. These cells are said to arise in the 

 anterior portion of the embryonic shield and to migrate along a very 

 definite route to the gut entoderm, thence through the splanchnic 

 mesoderm to the future gonad. A very similar later history has been 

 described by Woods (1902) also for the dog-fish, Squalus acanthias. 



Beard and Eigenmann agree that the germ-cells do not divide for a 

 long period in the early development of the embryo, that the original 

 number remains constant except for a slight diminution due to degen- 

 eration, and that a few may become stranded in the migratory process 

 in locations outside of the genital glands. 



The most detailed and satisfactory study of the germ-cells in the 

 chick has been made by Swift (1914, 1915, 1916) with the aid of 

 mitochondrial technics. He describes the origin of the primordial 

 germ-cells in a crescentic antero-lateral area from germ-wall entoderm 



