EMBRYOGENESIS IN THE ALGAE 47 



cell divisions in the longitudinal planes, Fig. 11b-e, the segmenta- 

 tion pattern being not unlike that found in some archegoniate and seed 

 plants. After some time growth is continued by means of an inter- 

 calary meristem. 



THE FUCALES 



The Fucales have provided classical materials for studies of the 

 embryogeny of seaweeds. Indeed, more is known about the ova and 

 young embryos of these plants than of any other group. Accordingly, 

 the main points in the development of Fucus and related genera will 

 here be considered in some detail (Figs. 3, 4, 11). 



The Ovum at, and after. Fertilisation. The eggs of Fucus and related 

 genera have afforded botanists materials which are comparable in many 

 respects with the eggs of various amphibians extensively used in 

 zoological studies. The spherical egg of Fucus, with an average diameter 

 of 75/Li in F. furcatus, and a nucleus which usually occupies a central 

 position, is similar in size to a number of sea urchin and other marine 

 invertebrate eggs. The eggs contain brownish green photosynthetic 

 plastids and are naked before fertilisation ; but immediately after there 

 is a secretion of mucilage which hardens to form a close-fitting, 

 cellulose-like wall. 



Levring (1947, 1949, 1952) has advanced our knowledge of the 

 submicroscopical structure of the eggs of the Fucaceae by using the 

 phase-contrast, polarising and centrifuge microscopes and various 

 special techniques. He has shown that the surface of a mature, un- 

 fertihsed egg consists of the following layers: (i) a gelatinous outer 

 coat, stratified tangentially, which seems to be partly dissolved by 

 water; (ii) a very thin egg membrane, which is of primary importance 

 in the subsequent formation of ihQ fertilisation membrane; (iii) a \v^o- 

 pvote'm plasma membrane; and (iv) an innermost cortical layer, which 

 produces the materials for wall-formation after fertilisation. Levring 

 agrees with other observers that the eggs of Fucus, both before fertili- 

 sation and in their post-fertilisation behaviour {see p. 20), are remark- 

 ably like those of the sea urchin. Both before and after fertilisation, 

 the egg is in a state of active metabolism. 



The penetration of the spermatozoid into the ovum and the rapid 

 nuclear fusion which follows — a matter of minutes according to 

 Farmer and Williams (1898) — set in motion a sequence of biochemical 

 changes. Within a few minutes a fertilised egg secretes a surrounding 

 membrane, whereas an unfertihsed one does not. If fertilised and 

 unfertilised eggs are placed in fresh water, the former burst at one 

 point, the contents being extruded through the hole, whereas the latter 

 do not burst but simply swell up and become altered in appearance. 



