230 



over the white hemisphere, as Pflüger ('83) thought was possibly 

 the case. 



I find that the extracrate cannot be regarded as necessarily a 

 fixed point. In some instances, it seems to be such — in others its 

 final position can only be explained on the assumption that it has 

 shifted its position. This uncertainly as to the fixity in position of 

 the ovate is a radical fault in the method, and gives to the con- 

 clusions only the value of probability. Certainty can only be reached 

 by studying the cell movements in the living egg, normally placed. 

 The photographic method employed by Kopsch ('95) is attractive in 

 its refinement, but from my own experience in observing the cell 

 movements in inverted eggs, I am inclined to believe that either in- 

 stantaneous photographs must be taken at intervals of a minute (the 

 interval Kopsch's experiments was much longer), or that the egg 

 must be kept under direct and practically continuous observation. 



My observations on compressed and inverted eggs show that in 

 eggs so placed the direction of cell proliferation is such as to shift 

 the position of cells, in the neighborhood of either dorsal or ventral 

 lip, towards that lip. Eycleshymer ("95) likewise has observed in 

 inverted Amblystoma eggs that small cells disappear round the dorsal 

 lip of the blastopore; and that when the blastopore has become cir- 

 cular and about Vs the diameter of the egg, the disappearance of 

 small cells round the ventral lip may also be observed. The centre 

 of the black hemisphere is probably a neutral region. Jordan's 

 observations ('93) on the rolling of cells, in the normally placed 

 Amblystoma egg, round the dorsal lip of the blastopore; and Eycles- 

 hymer's observations ('98) on the approach of ovates to, and their 

 disappearance (or that of the scar) round the dorso-lateral part of the 

 lip in Acris and Amblystoma, indicate that in the normally placed 

 egg, cell proliferation takes place from the apical (neutral) region 

 at any rate towards the dorsal lip. Some of my own experiments 

 indicate the same with respect to both dorsal and ventral lips. On 

 the other hand though the direction of cell proliferation be of this 

 character, it is yet possible that an ovate (or as in Schultze's ob- 

 servations, a natural mark) placed at a certain distance from either 

 lip, may approximately retain this position, as the result in part of 

 its own motion towards the lip, and in part of the backward growth 

 of the lip. This I believe to have been the case in some of my ex- 

 periments. 



If it is only the posterior portion of the neural plate that is 

 formed ])y the overgrowth of the dorsal lip, the concrescence theory 



