II MORPHOCHORETIC PATTERN CLEAVAGE 33I 



where the eggs divide into polar and antipolar halves. This is really a differential 

 cleavage, for the antipolar blastomere contains more RNA granules, which insure 

 the preservation of the full length chromosomes; while, in the other cell, they 

 undergo "diminution". The cleavage goes on with some spiral deviation, and 

 again one daughter cell of the antipolar half is favored in the same way as in the 

 preceding cycle; this goes on until the primordial germ cells are formed, with 

 their abundant RNA granules and long, whole chromosomes (Fig. ig; Pasteels, 



8 ~ 9 



Fig. 22. Localization of acidic phosphatases in the rat egg. The oocyte contains only a few 

 scattered granules. After fertilization, the enzyme is present in two forms, a diffuse one 

 (crossed areas) and a granular one (dots). Generally speaking, the diff"use reaction appears 

 in the areas more rich in RNA (compare with Fig. 1 7) while the granular reaction is linked 

 with the MPS-rich organelles. 2-5, cleavage stages; 6-7, a young and a more advanced 

 blastocyst; 8, cylinder stage; 9, young embryo. From 2 to 8 the dorsal part is orientated 

 toward the right of the figure. From Mulnard, 1955. 



1948). In Cyclops eggs, a differential distribution of RNA granules can also be 

 shown; this probably paves the way for differentiation of the primary gonocytes 

 (Stich, 1950). 



It is well-known that in the so-called mosaic eggs, specialized zones of cytoplasm 

 are formed by ooplasmic segregation (Costello, 1945) and are displaced after 

 fertilization and during cleavage. These processes have been most accurately 

 considered by Lehmann and his students on Tiibifex eggs (Lehmann and Biss, 



Literature p. 483 



