132 CHARLES EUGENE JOHNSON 



The two lateral cell-masses, representing the first head somites, 

 accord perfectly with Filatoff's account of the early stages of 

 these somites in Emys. No cavities are as yet to be found on 

 either side. The cells have no definite arrangement nor are they 

 individually distinguishable from ordinary mesenchymal ele- 

 ments. Mitotic figures are numerous and it is evident that the 

 mass is in active proliferation, growth in size taking place both 

 by addition from the wall of the foregut and by cell division 

 within the mass itself. A carefully made wax reconstruction of 

 the region involved shows the first head somites as two quite 

 similar, somewhat irregular rtiasses, which press out on each side 

 from the narrow space enclosed between the floor of the fore- 

 brain vesicle and the wall of the foregut. In each, the large 

 anterior portion narrows somewhat postero-medially towards its 

 origin in the outpushing entodermal cells of the digestive tube. 

 The upper sides of the two somites form a trough-like hollow, 

 closely surrounding the floor and sides of the prosencephalon. 

 Anteriorly they reach to the tip of the neural tube. 



The second and third head somites in Chelydra lie in a portion 

 of the head mesoderm which is a direct continuation forward of 

 the dorsal mesoderm of the trunk and occipital regions. They 

 answer, according to Filatoff, Dohrn's^ requirements for a true 

 somite in that their cells have a typical radial arrangement. 

 Filatoff points out, however, that Dohrn's definition is inade- 

 quate in that it does not allow for variations in the form of head 

 somites in the ascending zoological series, due to the modified 

 conditions of their development, and resulting perhaps in com- 

 plete obliteration of their cavities and a disarrangement of the 

 typical, radial cell order. In Chelydra the second head somite 

 shows the very marked radial cell order, but a clear cut lumen 

 cannot, at this stage at least, be claimed for it. It is probable 

 that if a sufficient number of embryos of approximately the same 

 developmental stage were examined, a lumen would be found to 

 exist. In the present investigation but one other embryo of 



''Dohrn, a. Studien zur Urgeschichte des Wirbelthierkorpers. Mittheil. aus 

 der Zool. Station zii Neopel. Bd. 15. 



