84 ARTHUR ROBINSON, M.D., AND RTCHARD ASSHETON, M.A 



with camera drawings of a complete series of transverse sections. 

 Our work was greatly facilitated by Professor Marshall, who very 

 kindly placed at our disposal a large series of sections, and to him 

 also our thanks are due for much kind advice. 



Before proceeding to the description of our own observations, we 

 must refer shortly to previous records, for by this course alone shall 

 we be able to indicate clearly the differences between our results and 

 those of the observers who have preceded us, and at the same time 

 we shall obtain an opportunity of defining some of the terms that we 

 shall be obliged to use in our description. 



Turning, therefore, in the first place to the consideration of the 

 formation of the archenteron and the blastopore (but leaving aside 

 for the present the concrescence theory of His [22 and 23], so far as 

 it concerns the formation of this cavity), we find that upon this, as 

 iipon most other important points, there is a distinct difference of 

 opinion between the previous observers. It is stated by some that 

 after the formation of the segmentation cavity, and when the ovum is 

 only partially covered by the pigmented epiblast-cells, the archenteron 

 is formed by an invagination, which "first commences by an inflection 

 of the epiblast-cells for a small arc on the equatorial line which marks 

 the junction between the epiblastic cells and the yolk-cells " 

 (1, p. 102). 



This preliminary invagination is also described by Perenyi in 

 Bombinator (40), 0. Schultze in Rana fusca (45), and by Scott and 

 Osborn in the Newt (48). 



Whilst it is proceeding the enclosure of the yolk has been rapidly 

 taking place. " It is effected by the epiblast growing over the yolk 

 at all points of its circumference" (1, p. 102). We have in this 

 latter statement of Balfour's a very fair summary of the general 

 opinion that the epiblast and yolk are distinct parts of the ovum ; 

 indeed, Balfour speaks of the yolk-cells as " these large cells which 

 are part of the primitive hypoblast" (1, p. 101), with which, in our 

 opinion, they cannot fairly be compared ; and we shall speak of them 

 in the sequel merely as yolk-cells, though probably the more correct 

 term would be germ segments. It is not necessary, however, to enter 

 into a discussion of the mode of extension of the epiblast at the 

 present moment, and we can proceed, therefore, to a point which is 



