Ill 



EXTRINSIC MORPHOCHORESIS 



365 



(Mulnard, 1955). As already pointed out, the enzyme first appears in the 

 enveloped group of cells which soon transform into the ectophyll. During the 

 movements of the streak, the positive reaction partially decreases and parallels 

 the utilization of RNA; the immigrated mesoblast is quite feebly positive, with 

 a decrease from the presumptive somitic material towards the future lateral plate. 



Fig. 38. Synthesis of alkaline phosphatase in the apical ridge of the limb buds of the rat 

 (a,b) 13th day; the enzyme has only appeared in the caudal part of the ridge; none existed 

 at the preceding stage. The marginal veinous sinus runs just under the ridge; b) detail of 

 the positive region; (c, d) 14th day; longitudinal (c) and transversal (d) section. The whole 

 ridge now produces the enzyme. From Milaire, 1956. 



The ectoblast {i.e. the non-immigrating part of the ectophyll) becomes for a 

 while moderately positive. Then, the enzyme seems to disappear from the 

 epiblast (future epiderm), while it becomes quite abundant in the neural plate, 

 which is also known to elaborate ribonucleoproteins. 



One might ask why such a striking difference has not been perceived in 



Literature p. 483 



