26 Mr. A. H. Hassall on the Anatomy and Physiology 



view before expressed of its office in presiding over the develop- 

 ment of the cells, it is clear that this tubular apparatus is de- 

 signed to facilitate the transmission from zoospore to zoospore of 

 the elaborated nutriment transmitted by the cytoblast. The or- 

 ganic mucus and endochrome, from their appearance and limita- 

 tion in the genus Zygnema, may be supposed to have passed by 

 exudation from the interior of the tubes to their surfaces. See 

 %. 1,8. 3. 



On the Formation of Spores {Sporangia ?) and their investing 

 Membranes, — The following would appear to be some of the steps 

 in the formation of spores. The material, whether consisting of 

 the contents of one or two cells, out of which each spore is to be 

 formed, first consolidates itself in the centre of those cells in which 

 it is to be elaborated ; the zoospores, which continue to increase 

 in size, retire from the surface of the mass so as to leave only or- 

 ganic mucus surrounding them ; this then assumes the form of 

 spore peculiar to the species, its surface, lastly, becoming hard- 

 ened into a compact membrane or membranes. 



Observations on the genus Zygnema. 



The species of the genus Zygnema readily resolve themselves 

 into two divisions or subgenera, which are to be distinguished 

 from each other by the conformation of the cells. 



In the first of these subdivisions, which for the most part in- 

 cludes the long-celled species of the genus, such as Zygnema elon- 

 gatum and Z. quadratum, &c., the opposed extremities of all those 

 cells which have attained maturity are considerably inverted, and 

 which inversion may be compared to that of the finger of a glove 

 (PI. I. fig. 4) ; while in the second, which embraces the short-celled 

 examples, as Zygnema maximum, Z. nitidum and very many others, 

 the cells are not inverted, but touch each other by their plane 

 surfaces. 



The form of this inversion is, in all the species in which it occurs, 

 identical and extremely regular, its circumference being circular 

 and its base somewhat flat \ no membrane intervenes between the 

 spores formed by this indoubling in contiguous cells, which spaces 

 therefore communicate directly with each other. 



At the period of reproduction, and at no other, one of the two 

 indented and opposed extremities of certain cells becomes everted 

 and protruded into the cavity of the other (PL I. fig. 5). 



The cause of this protrusion, and the reason why it only occurs 

 at the precise period of the reproduction of the cells, are easily 

 accounted for, and both arise from unequal internal pressure of 

 the contiguous cells on each other, which inequality of pressure 

 is produced by the emission of the endochrome of one cell into a 

 neighbouring cell either in the same or different filaments j thus, 



