227 



and more crowded. By this time the position of the ventral and 

 lateral parts of the blastopore lip is indicated by the presence, round 

 the inner edge of the delamination zone, of the angular pigment 

 spots (depressions) described above. The spots at this time are 

 comparatively few and far apart, but by 4,30 P.M. they have in- 

 creased in number and united, and the blastopore lip is now de- 

 veloped as a distinct edge (furrow) all round. The blastopore at this 

 time is circular, and its diameter is roughly ^/^ that of the egg (dia- 

 meter of this egg is actually 100, diameter of blastopore 73). Its 

 position is nearly central, the ventral lip being slightly nearer the 

 -periphery of the image, than the dorsal lip. During the subsequent 

 narrowing, the transverse diameter of the blastopore becomes some- 

 what less than the dorso-ventral diameter (42:50 at 9,30 P.M.). The 

 blastopore continued to occupy a position, which was nearly central 

 in the image (the slight exceutricity would not have been noticed 

 in a lens examination over a mirror), until the 90 <> rotation began. 

 The belief in the occurrence, in the amphibian egg, of an ex- 

 tensive delamination from the randzone downwards, which was ex- 

 pressed by Robinson and Assheton in 1891, and recently by 

 Grönroos ('98), has been based upon a study of sections. This 

 reliance on sections makes the conclusion a freely contestable one. 

 Morgan in a brief note published in 1891 records the fact of his 

 having witnessed in surface views of (living?) eggs, the transform- 

 ation of yolk cells into epiblast ; but in his text-book ('97) he does 

 not refer to the process. 



Since it may be observed after the fashion above described, in 

 the normal egg, that the pigmentation in the zone of ectoderm lying 

 immediately above the just formed ventral and lateral hps, is de- 

 veloped in situ (white cells first developing chromatophores , and 

 then dividing and growing pigmented), it follows that the apparent 

 position which the blastopore, when first completely outlined as a 

 circle, occupies in a microscopic image (or mirror picture), is its real 

 position with respect to the rest of the egg. That is, during this 

 period of development (delamination period) no rotation of the egg 

 as a whole takes place; and the apparent movement of the dorsal 

 lip away from the periphery towards the centre of the image, is a 

 real movement (overgrowth) over the yolk. 



The extent of the overgrowth of the dorsal lip, between the 

 time of its own appearance and that of the ventral lip, may be cal- 

 culated with the help of a micrometer eye-piece and inverted micro- 

 scope. The actual measurements to be made, are the diameter of 



15* 



