EMBRYOGENESIS IN THE ALGAE 49 



fertilised egg can be regulated by a local application of auxin. When 

 an egg was placed at the end of a very fine capillary containing the 

 growth-substance in solution, the rhizoid originated towards the 

 capillary, and the first dividing wall was at right angles to it: in the 

 untreated controls, the rhizoid grew out at random. This and other 

 evidence indicates that auxin is both present in the fertilised egg and is 

 a factor determining polarity. The accumulation of auxin at some 

 particular locus in the egg is, of course, another problem. 



That metabolism is very active in the pre- and post-fertilisation egg 

 of Fucus is clearly shown by the data of respiration studies (Whitaker, 

 1931). UnfertiHsed eggs of Fucus vesiculosus in the dark consume about 

 5-2 cu. mm. of oxygen per hour per 1000 eggs at 18°C, a high rate as 

 compared with that of marine invertebrate eggs. With an illumination 

 of 100,000 foot candles, Fucus eggs liberate in photosynthesis more than 

 twice as much oxygen as they consume. There is a sharp increase in the 

 consumption of oxygen immediately after fertilisation (to 190 per cent 

 of the pre-fertilisation rate). This rate is maintained uniformly for 

 about 13-14 hours after which there is a slight increase until 24 hours. 

 At 18°C about 50 per cent of the eggs had undergone their first cleavage 

 after 13-18 hours — average 15 hours. These data suggest that, in 

 certain respects, the phenomenon of fertiUsation is essentially similar 

 in marine plants and animals. 



Information on the constitution of the fertilised egg in Fucus is 

 afforded by experiments in which eggs were subjected to unilateral 

 irradiation with monochromatic ultraviolet light and then placed in a 

 hypertonic sea-water-sucrose solution: 95-97 per cent of the eggs 

 showed a polarised plasmolysis, the non-irradiated half which would 

 normally have given rise to the rhizoid being affected. Reed and 

 Whitaker (1944) have suggested that when water is withdrawn the non- 

 irradiated half of the egg shrinks, the other half having been stiffened, 

 strengthened and rendered more viscous by the treatment. These 

 plasmolytic effects can be demonstrated considerably earlier than the 

 first appearance of the rhizoid, i.e. metabolic changes precede the 

 visible morphological development. The responsiveness of eggs to 

 irradiation and plasmolysis increases gradually after fertilisation 

 reaching a maximum after about 7 hours and remaining at that level 

 for about 3 hours. 



When fertilised eggs of Fucus furcatus f. luxurians were placed in 

 sea water in a salinity range of 60-150 per cent of the normal con- 

 centration, practically all formed normal rhizoids and continued to 

 develop. In sahnities greater or less than 90-100 per cent normal, 

 4-day embryos showed a reduction in their growth rate (elongation). 

 Below 60 and above 150 per cent salinity there is a rapid decline in 



