460 THE AMERICAN NATURALIST. [VOL. XXXVI II. 



respective plasma membranes and between them the so-called 

 middle lamella. 



The middle lamella has been the subject of much discussion. 

 It is not the remains of the cell plate as was once supposed. 

 Neither is it exactly a cement between two cell walls. Its his- 

 tory is undoubtedly various, for the composition shows much 

 plasticity. The origin of the middle lamella at the surface of 

 a plasma membrane indicates a morphology similar to a cell wall, 

 but the substance, pectic in character, shows transformations far 

 removed from the cellulose compounds that are formed later and 

 which give thickness to the cell wall. Allen (:oi) discusses the 

 subject in detail. 



The origin of the cell plate is a subject of interest which will 

 be further discussed in Section VI. There are some types, espe- 

 cially among the thallophytes, where a cell plate is present, but 

 apparently in a somewhat undeveloped and rudimentary condi- 

 tion. These forms suggest transitional conditions between cleav- 

 age by constriction with the aid of vacuoles, so general among 

 the thallophytes, and cleavage by the cell plate, characteristic of 

 higher groups. The most interesting examples are Anthoceros, 

 Chara, Basidiobolus, Pelvetia, Fucus, and Sphacelaria. 



Cell plates are formed with each of the two successive mitoses 

 in the spore mother cell of Anthoceros (Van Hook, : oo ; Davis, 

 :oi, p. 158), but the structure in some species is exceedingly 

 small (e. g., A. Icevis} and can scarcely extend more than one- 

 tenth of the distance across the cell. It is larger in other forms, 

 as in the one studied by Van Hook ; but even there the nuclear 

 figure of the second mitosis is only one-third of the width of the 

 cell. The protoplasm divides simultaneously in the four spores 

 with the characteristic arrangement. If this division were deter- 

 mined entirely by cell plates there would be required an exten- 

 sive development of fibrillae, of which there is no evidence in the 

 cell. But their place seems to.be taken by numerous delicate 

 strands of cytoplasm which connect the four protoplasmic masses, 

 each of which contains a large chromatophore and an accom- 

 panying nucleus. A film is formed in the intermediate region, 

 and this marks the position of the cell wall. It is, of course, 

 quite certain that the two cell plates of the second mitosis are 



