102 SCIENCE PROGRESS 



genus, and the bivalved character of the wall of the Phacotaceae 

 (Chlamydomonadineae). The peculiarity of the sexual process 

 of Chlauiydomonas Braunii {s&q first part of this article, fig. i, g, 

 p. 629) lies in the fact that the clothed gametes become united 

 by fusion of their walls at the apex, after which the contents 

 of the microgamete pass through the canal thus established into 

 those of the macrogamete, a process very similar to the con- 

 jugation of a Spirogyra. It is quite possible that in forms ex- 

 hibiting this type of conjugation the power of free movement (by 

 cilia) of the gametes might be given up altogether, simultaneously 

 with its disappearance in the ordinary vegetative individual, and 

 especially if a filamentous tendency set in. In some such way 

 the Conjugatae may have arisen. Although the character of the 

 sexual process and the lack of motile power in the gametes is 

 thus not a point of insuperable difficulty, the bivalved nature 

 of the wall in the Desmids (one of the two large subdivisions 

 of the Akontae) is not easily paralleled (Lutkemuller 48). The 

 Phacotaceae (cf. p. 631 in the first part of this article) show a 

 similar feature, but the valves there do not fit over one 

 another as they do in the Desmids ; nor is the method of cell- 

 division at all similar in the two cases, for in the latter each 

 daughter-cell receives one valve of the mother-cell and only 

 forms the other half afresh. The method of cell-division and 

 the structure of the cell-wall in certain Heterokontae (like Con- 

 ferva) and in the (Edogoniaceae shows some analogies with 

 that in Desmids, but they are at the best rather remote. The 

 Akontae, therefore, occupy much the same sort of position at 

 the present moment as do the Stephanokontae, but the evidence 

 for their direct origin from the Isokontan stock is rather more 

 weighty than in the case of the latter group. 



The Conjugatae are usually divided into two series, the 

 Zygnemoideae and the Desmidioideae {e.g. Blackman and Tans- 

 ley 6). Of these the former consist of unbranched filamentous 

 forms (e.g. Spirogyra, Zyg}icma, Moiigeofia), while the latter are 

 prevalently unicellular, although there are a number of fila- 

 mentous representatives. The Desmids are also peculiar in the 

 bivalved character of the cell-wall referred to above, and in the 

 perforation of the wall by numerous pores through which mucilage 

 is excreted (Schroder 61, Lutkemuller 48). Both series agree 

 in the possession of large and usually complex chloroplasts often 

 provided with numerous pyrenoids. There is much difference 



