EMBRYOGENESIS IN THE ALGAE 51 



Stratification effect when eggs develop in close proximity at pH 60 and 

 that auxin (indole-3-acetic acid) in the medium at pH 6-3 and 8-0 

 tends to promote rhizoid formation at the centripetal pole. 



Whitaker has also explored the effect of mechanical factors on the 

 growing Fucus egg. When fertilised eggs of F. fur cat us were suclced into 

 a small pipette while the cell wall was still hardening, the elongated 

 shape which they assumed was retained when they were released into 

 sea water. In normal sea water, at pH 7-8-8-2, the rhizoids grew out 

 at or near one end of the long axis, and cell division took place at right- 

 angles to this axis. The shape imposed on the zygote thus determined 

 its polarity. But when similar zygotes were released into sea water 

 acidified to pH 6-0, rhizoids were typically formed towards the base of 

 the dish, i.e. where the concentration of diffusates from the eggs would 

 be greatest. A biochemical effect is thus able to overcome the 

 mechanically-imposed shape effect in the determination of polarity 

 (Fig. 4). 



Not only are there well marked differences in the oogenesis of 

 different genera, but this also holds for the size of the ova, the time 

 required for fertilisation (e.g. 15-16 minutes in Halidrys as compared 

 with a few minutes for Fucus), and their reactions during and after 

 fertihsation. These differences are indicative of differences in the 

 respective genetical constitutions. Some of these may be of a far- 

 reaching kind. For example, in Fucus the egg is naked and free- 

 floating, whereas in Pelvetia and several other genera it remains en- 

 closed within the oogonial wall, and actually undergoes its early 

 embryonic development therein (Thuret, 1854; Fritsch, 1945, p. 372- 

 374). In Sargassum and Bifurcaria the extruded oogonia remain for 

 several days attached to the inside of the conceptacle by means of long 

 gelatinous stalks {see p. 54). 



Early Embryonic Developments. In Fucus, Ascophyllum and Pelvetia 

 the division of the zygote nucleus is followed by the formation of the 

 first transverse wall. In Himanthalia it is otherwise, several nuclear 

 divisions taking place before segmentation begins {see Figs Id; 2b; 

 3d; 6e; 11f-l). 



The unfertilised (or newly fertilised) ovum either has no definite 

 polarity or a very labile and readily alterable one. As we had seen in 

 Chapter III, polarity may be determined by such factors as the entry 

 of the spermatozoid or other random disturbances, by unilateral 

 illumination, gradients of electrical potential, temperature, pH, auxin 

 or other substances diffusing from the zygotes, and by the redistribution 

 of materials effected by centrifuging. It may be assumed that the 

 materials in the zygote are initially symmetrically distributed about the 

 central nucleus. In relation to one or more of the factors indicated 



