i8 



CLEAVAGE AND DIFFERENTIATION 



and the prelocalisation of organ-forming substances appeared to 

 be of considerable theoretical importance. From the point of view 

 here adopted, the absence of prelocalisation in such cases is seen 

 to be a frequent (though not universal) consequence of precocious 



>»^'-'^P 



C 



Oy-i- 



Fig. 56 



Localisation of organ-forming substances in Ascidians. Views of eggs of Styela. 

 Yellow cytoplasm containing mitochondria {yp) small circles. Yolk {Gy) 

 stippled. Clear cytoplasm {cp) white. A, Before fertilisation, showing germinal 

 vesicle {GV), yellow cytoplasm evenly distributed over the surface. B, Imme- 

 diately after fertilisation, showing clear region {kp) derived from germinal 

 vesicle at animal pole, yellow cytoplasm streaming down to vegetative pole. 

 C, The yellow cytoplasm forms a cap at the vegetative pole {yz) containing the 

 male pronucleus. The clear cytoplasm forms a layer just above it. D, Left side 

 view of egg just before first cleavage, showing yellow crescent {yc) and clear 

 crescent {cp) posterior, and grey crescent (Gc) anterior. (From Conklin, 

 Chap. IX of Cowdry, General Cytology, Chicago, 1924.) 



chemo-differentiation. When chemo-differentiation occurs prior to 

 fertilisation, the differentiated substances thus produced are able to 

 shift their relative positions, either in the uncleaved tgg, or as a 

 result of manoeuvres effected during cleavage. If, on the other hand, 



