INTRINSIC MORPHOCHORESIS 



321 



general pattern sketched in the mature egg can orientate the events of the early 

 stages, and how far these events have already been understood. We have to 

 consider therefore which changes may occur at fertilization, the relation of 

 cleavage to the primary egg pattern, and how and why cell movements appear 

 and result in a discrete diversification of the general system. 





Fig. 12. Successive stages (stained sections of fixed specimens) of "sorting out" occurring in 

 intermingled epiblast and neuroepithelial cells in an Amphibian. A piece of neural plate 

 (without archenteric roof) and one of epiblast have been excised, and disaggregated by 

 means of alkali. Under readjusted conditions, the cells reaggregate (a) and subsequently 

 segregate (b, c, d) so that the surface of the explant becomes epiblastic and a brainlike 

 mass is reformed inside. From Townes and Holtfreter, 1955. 



B. Fertilization and morphochoretic pattern 



As strange as it may seem, the influence of fertilization upon the egg pattern 

 is only known — and not yet exhaustively — for one animal group, the amphibians. 

 There exist also some cases in which the ordinary point of penetration is known, 

 as in Ascidians or in Insects, but in which there is no indication, so far, of any 

 eflfect on the pattern. 



Amphibian eggs have been known for a long time to show, shortly after 

 fertilization, a semi-macroscopical picture which permits recognition of the future 

 dorsal side. This is due to a general "upwards" movement of the cortical layer and 

 its covering pellicle in this region, which pulls the pigment up after it (Fig. 13). 

 Simultaneously, a slight ascension of the somewhat modified yolk mass takes place 



Literature p. 483 



