MORPHOCHORETIC PATTERN CLEAVAGE 



333 



then thoroughly washed in sea-water. They immediately show a gradual metachromasy, 

 due to the staining of numerous, just microscopically visible, granules which Pasteels has 

 labelled a. In the sea-urchin Paracentrotus these granules also stain by neutral red. A cortical 

 film, corresponding to the vitelline membrane, i.e. the plasmolemma of the cell, is always per- 

 fectly unstained. In some eggs {Spiralia) a layer of larger granules which are also seen in the 

 cortex at early stages are metachromatic. They correspond to F. R. Lillie's cortical granules 

 and remain unchanged during cleavage. Their metachromasy is due to mucopolysaccharides. 



The true a-granules are evenly distributed in the cytoplasm, except in Ascidiella where 

 at first they are mostly located vmder the plasmolemma of the unfertilized or just fertilized 

 egg. In this special case, they flow towards the antipole, next to the "yellow" plasm, and 

 soon are caught in the rays of the spermaster (Fig. 29, a-d). 



Everywhere, centrifugation shows that the a-granules (certainly not an artefact produced 

 by the vital staining) are relatively heavy, sometimes even heavier than the yolk platelets 

 (Fig. 27, a, b, c). After fixation, they stain by the PAS technique, even after enzymatic 

 digestion, but not by alcian blue (at a low pH) unless the sections have previously been 

 submitted to a strong oxydation (Fig. 28, a, b) ; RNA seems to be absent or scarce; reactions 

 for acid phosphatases (Fig. 28 d) are positive except for the case of Parac^w^ra^Mj eggs. Thus 

 these granules appear to be mostly built up of neutral or feebly acidic mucopolysaccharides, 

 and a protidic moiety which acts as a phosphomonoesterase on the acidic side of the pH scale. 



After a certain delay, a second category of granules, Pasteels' [3, becomes apparent. 

 They are distinctly larger (often more than i[i.) and less regular, also less numerous, 

 mostly located in the perinuclear or perifusorial area (Fig. 23; 28, c) where their presence 

 in unstained eggs {Barnea) can be ascertained by phase contrast examination. In Spiralia 

 eggs, where the staining can be performed on oocytes, some granules already exist around 



(text continued on p. 336) 



Fig. 23 Metachromasia in vivo in a Barnea cand. egg (Lamellibranch). The fertilized eggs 

 were dipped for 5 min. in toluidine blue 1/30,000 in sea water, then carefully washed 

 and the development observed. Specimens were fixed at successive moments in Izquierdo's 

 fluid and niounted. These drawings show only the ^-granules, appearing mainly near the 

 pronuclei and increasing in number until the first cleavage, where they gather inside the 

 asters. In the 2-cell stage, the larger cell (CD) contains more of these granules. The cycle 

 begins again at each cleavage, the diffuse a-granules transmitting the dye to the still more 

 numerous fi bodies. The D cell is the richest in [3-granules. The test for acidic phosphatase is 

 positive at any of these stages, in these granules and also in the yolk. From Pasteels and 



Mulnard, 1957. 



Literature p. 483 



