304 THE DEVELOPMENT 



the yolk. Upon close examination it will be found that this disc 



is b u t the thicker portion (fig. 

 204, gV) of a layer (g-Z 1 ) of cells 

 which has been formed all over 

 the periphery of the yolk (//). 



' ff^EIR H^HI r '""''~ layer is called the germinal 



layer, and the thicker, disc-shaped 



^^Q|^99H^^9fi^^HI portion of it the embryonic disc. 



Owing to the influx of a portion 

 of the liquified albumen, the re- 

 gion (ft/ 1 ) immediately beneath 

 F'g-204. the disc is very transparent at 



this time ; and, by throwing a strong ray of light through it, we 

 may get such a profile view of the various parts as are repre- 

 sented in this figure (fig. 204). 



One of the chief elements in the formation of the organs of 

 the young turtle is the development of what I have called the 

 subsidiary layer* This is a stratum (sl l ) of loose yolk granules 

 which originates at the surface of the yolk, and extends all 

 around it. I have called it the subsidiary layer, because it is a 

 sort of intermediary between the unappropriated yolk and the 

 definitely fixed parts of the growing organization ; in fact, as I 



Fig. 204. Chelyilra serpentina. Schweig. 2 diam. A diagramic, profile 

 representation of the incipient germinal disc of the common snapping turtle, as 

 it lies within the egg. al, the white of the egg ; y, the yolk ; ?/', the surface of 

 the yolk ; vs, the periphery of the yolk ; a/ 1 , the clear, albuminous region beneath 

 the disc ; gl, the disc ; gl l , the germinal layer ; si, sl l , the subsidiary layer. 

 Original. 



* See my observations to this effect in Agassiz's " Contributions to the Nat- 

 ural History of the United States," vol. ir. p. 536. I would add, in reference 

 to the " European naturalists " mentioned in the " Note on Scientific Property," 

 p. 38, see Kolliker, " Entunckelungxgeschichte, 'e., Akademie Vor'rage" 1861, p. 

 20 ; Gegenbaur, " Ban fy Entwickl. WirbeltJiiereier" Archiv.fur Anat. und Pliy- 

 siol., 1861, p. 497 ; and also Leuckart, " Berichte " in Wiegman's (TrocheUs) Ar- 

 chiv. filr Naturgeschichte, 1864-5. From American scientific men I have re- 

 ceived abundant recognition of my claim, both by correspondence and in public 

 lectures ; to say nothing of their free and outspoken sympathy in personal in- 

 tercourse. 



