229 



becomes fairly distinct, the light colored neural area is found to ex 

 tend slightly in front of the flattened part of the upper surface, and 

 to include 120» (or something over). At its sides and anteriorly, 

 the pigmented border soon becomes elevated as a low rounded ridge.' 

 The length of the neural plate, not including the anterior connective, 

 is thus at the start fully 120". The measurements were made on 

 numerous live eggs with micrometer ocular, and confirmed by meas- 

 urements of median longitudinal sections of preserved embryos. 



If the Roux-Hertwig- Morgan theory is correct, the ventral 

 lip of the blastopore must remain stationary, while the dorsal lip 

 travels 120° over the yolk, thus providing the material, out of which 

 tlie neural plate is differentiated. 



Pricking Experiments. 



Pricking experiments on the amphibian egg have led in different 

 hands to very different conclusions. Roux and Morgan are con- 

 vinced that the ventral lip remains stationary (or practically so, 

 Morgan), the dorsal lip travelling over the yoik to it (180« Roux,' 

 1200 Morgan). Assheton ('94) finds that both ventral and dorsal 

 lips overgrow the yolk, the dorsal lip moving through about 70"— 

 that is from spot a little below the equator to the lower pole, or 

 perhaps a little beyond it". Eygleshymer ('95, '98) reaches sub- 

 stantially the same result: "the greater portion of the embryo arises 

 in the darker hemisphere by differentiation in situ, and not by con- 

 crescence" ('98). Schulze in his admirable 1890 paper, gives it as 

 his experience that such (apparently) contradictory results are reached, 

 as to make the method unsuitable for the solution of the problem.' 



Schultze's observations on the position of natural marks on the 

 egg surface, indicate that at any rate a part of the neural plate is 

 foi-med on the black hemisphere — an examination of his figures, 

 e. g. Figs, la, lb. Ilia, 111b, shows I think that it is not necessary 

 to conclude with Schultze that the entire neural plate is formed 

 on this surface. 



My own experiments led to such apparently contradictory results 

 that I was inclined to take Schultze's view of the method. After 

 comparing and classifying my experiments however, it seems to me the 

 results are all explicable on the theory advanced by Assheton ('94), 

 Whitman and Eycleshymer ('95), that dorsal and ventral lips over- 

 grow the yolk, from the places of their first appearance to the lower 

 pole - the neural plate hence being formed in part on the black 

 hemisphere, and in part by the backward growth of the dorsal lip 



