94 



K. MIT«UKURI; 



with, or perhaps even before, the commencement of gastrnlation. In 

 fact, I am inclined to think that the e.stabhshment of the epiblast at the 

 future dorsal lip of the blastopore is the signal for tlie commencement 

 of the invagination {see Contrib 7, p. 236). If we also look at Will's 

 figures 42, 44, 48, ('92), or loa, 13/), 16 ('93), tlie above objection 

 will not h(3ld. It seems to me that the explanation just given of the 

 nature of the primitive plate or lônoh is a ^'ery satisfactory one, and has 

 the advantage of deriving this structure from something existing 

 already in Icthyopsida. 



According to this view, then, the 



Woodcut XVI. , , . , ^ . . 



cheloman eo-g mav l)e represented as in 



Pr. Kn. or im. 1. '-^'- ■• '■ 



AVoodcut XVI. The primitive knol:) (jr 

 plate (Fr. Kit.) represents the primary 

 volk-mass and is homologous with the 

 lower laroer-celled half of the amphibian 

 ovum and with the large yolk-mass 

 (including all the parts that do not 

 l)elong to the epiblast) of Elasmobranchii. 

 The larger yolk-mass (Yiii. 2) of Chelonia, 

 although developed in conti nr.ity with 

 the pri Diary yolk-mass, must be looked 

 on as S(3mething newly acquired and not 

 represented in the ovum of Elasmobranchii 

 or of Amphibia. The hypoblast surrounding it may also be considered 

 as cœnogenetic. The problem;itical cell mass, whicli I have all along 

 called the yolk-plug, is directly derived from a |)art of the primitive 

 plate after the process of gastrnlation has taken ]>lace in the primitive 

 plate. If the primitive plate corresponds to the lower larger-celled 

 half of the amphi])ian ovum before gastrnlation. the ])roblematical 

 cell-mass in such a stage as Fig. 1, PI. I. corresponds to the posterior 



Diagram of a cheloniari egg. 



Pi-. A'h. = Primitive knoli) or plate. 

 Ym. i = Primary yolk-mass. 

 Ym. ;2 = Secondary yolk-mass. 



