II MORPHOCHORETIC PATTERN CLEAVAGE 339 



revealed the [5-granules by vital staining with methylene lilue, but concluded from 

 his E.M. studies that he was dealing with mitochondria severely modified by the 

 staining treatment. However, one knows it is always a delicate problem to establish 

 a correspondance between microscopic and ultra-microscopic appearances. 



Finally, we could ask what is the possible function of the metachromatic 

 particles. At first sight, there are several hints of some relation between the 

 distribution of p-granules and early morphogenesis. In mollusk eggs, an excess 

 of ^-granules endow the D-quadrant or eventually the polar lobe. In sea-urchin 

 eggs, the micromeres when they appear, as well known, at the antipole during 

 the 4th cleavage, are practically devoid of ^-granvdes. Later on, these bodies 

 surround the nuclei of the morula, except in the micromere derivates. In the 

 swimming blastula, they form a continuous layer at the base of the cilia, possibly 

 in coincidence with the parabasal bodies. In the gastrula and young pluteus, the 

 stain is liberated and condensed into vacuoles. This process of clearance first 

 takes place in the mesenchyme, then in the ectoblast, except its thicker parts 

 which prepare the arms and the oral plate, still later in the archenteron. In rat 

 eggs, metachromatic granules become most conspicuous at the 8-cell stage and 

 are more abundant in the 4 large "ventral" cells, which will envelop the 4 smaller 

 ones and form the enveloping layer (Fig. 21), devoted partly to the endophyll 

 (p. 319) and mostly to the trophoblast. In Ascidians, the metachromatic granules 

 show a tendency to be associated with the myogenic plasm (Fig. 29, a-f), and 

 indeed, in the tadpole, the myoblasts of the tail remain the more distinctly meta- 

 chromatic (Fig. 29, g, h). The diflfuse metaplasm is linked with the hind part of 

 the enteron (Fig. 29, a-h). 



Considering these relations more closely, it may be remarked that the abundance 

 of ^-granules in the D-quadrant of Spiralia-eggs may depend on the well-known 

 inequality of the mitotic poles at the first and second cycles. If this is the case, the 

 distribution of the p-granules would thus be secondary and depend on the funda- 

 mental organization pattern of the egg. It may however be linked with the 

 recognized abundance of mitochondria in the somatoblasts {cf. p. 331). In sea- 

 urchin eggs, the scarcity of metachromasia in the micromeres may simply be a 

 consequence of the rather deep location of the [^-granules. This is contrary to the 

 idea of a positive influence by the p-granules on the course of morphochoresis, 

 and, thereby, on morphogenesis, for it has indeed been established by Horstadius 

 that the micromeres possess the highest degree of "vegetalizing " activity^ . Moreover, 

 Brice (1959b) has observed that, in the same eggs, metachromatic staining inter- 

 fers with neither animalization nor vegetalization. Again, stained eggs remain 

 capable of regulation after separation of the first blastomeres. If the a- and [3- 

 granules had a positive role in these processes, it seems likely that their becoming 

 loaded with dye cations would not have an indifferent effect. The sequence of 

 clearance in the young plutei may be related to the size of the cells, their location, 

 a.s.o. Thus, in sea-urchin eggs, the evidence is not in favour of a direct morpho- 



^ There are definite hints that mitochondria are involved in the egg pattern of gradients. 

 In the morula and blastula, they are distinctly more numerous in the animal half than in 

 the vegetative one (Horstadius, 1952; Gustavson and Lenique, 1952; Lenique, Horstadius 

 and Gustavson, 1953). 



Literature p. 483 



