3l8 GERMINAL ORGANIZATION - — INDUCTION PHENOMENA 4 



a villous appearance (Fig. 6, b, p. 31 2) which is probably the expression of the 

 intense activity of this critical interface. The cortical layer includes this film 

 plus a variable underlying layer, partly homogeneous, partly crowded with 

 granules and quite difficult to displace by centrifuging. It seems that this cortical 

 layer is the most stable agent of polarity. Not only does it explain why a centri- 

 fuged egg is able, after a sufficient lapse of time, to restore the typical distribution 

 of its materials, but it also accounts for the special segregation which takes place 

 in certain eggs with spiral cleavage. Among the species (Schleip's Spiralia, 1928) 

 which show this characteristic feature, the one which has perhaps been the most 



^ .-.-«»f|i»«'--. 



4-J 



0.1mm 



Fig. 1 1. Ordered and polarized changes of form in the egg of Tiibifex, during the formation 

 of the first polar body, (a) Metaphase of the first maturation spindle; (b) early anaphase 

 with the outer pole bulging at the surface; (c) advanced anaphasis with the pinching off 

 of the polar body; (d) telophase. First vertical row: the whole egg, seen from its animal pole 

 at the corresponding stages. Second vertical row, lateral view of the same. Third row shows 

 the progress of maturation. From Rotheli, 1950. 



thoroughly investigated is Liinnea; for this mollusk egg also, the only recognised 

 basis of a possible explanation is that polarity is in some way inscribed in the 

 cortex (Raven, 1948, 1958). A quite simple observation reveals this stabilizing 

 influence of the cortex. If a time lapse film is taken of the rupturing nucleus in 

 the transparent oocyte of the bivalve Barnea Candida (records of Pasteels and 

 Mulnard, 1957), such a complete whirl of the internal cytoplasm appears that 

 it is impossible to imagine any definite structure or stratification which could 

 exist there and be preserved. And yet, maturation and all further events take 

 place according to the preexisting polarity of the oocyte. That the formation of 



