750 PHYSIOLOGY. 



the structure of the conjugating cells, previous to impreg- 

 nation. 



Two methods of conjugation may be noticed, one, which oc- 

 curs generally in the Diatomace£e, and the other in certain of 

 the Chlorosporeas. In the first mode ( figs. 1096 and 1101), two 



Pi^. 1101. 



Fig. 1101.— Two Desmidiaceous AlgjE {Docidivm Ehrenhergii), in process of 

 conjugating. The contents of the two are seen to be intermingling in the 

 intervening space, and are at present only invested by a primordial utricle. 

 After Ralfs. 



individuals, each of which is composed of a single cell, approach 

 one another, the external cellulose membranes bounding their 

 respective cells bui'st, and the contents of the two, invested by 

 a primordial utricle, issue from the orifices thus produced {fig. 

 1101), intermingle in the intervening space, and form ulti- 

 mately, by their mutual action, a rounded body {fig. 1096), 

 called a resting or inactive si)ore, which ultimately germinates. 

 The contents of the spore are green and granular at first, but 

 ultimately become brown, yellow, or reddish. These resting 

 spores, which are furnished with two coats {fig. 1096), are 

 sometimes called sporangia, because they ultimately pi'oduce 

 two or more germs in their interior, and are not therefore simple 

 spores. 



In the other mode of conjugation, which occurs in certain 

 Chlorosporcaj, as in Zygnema {fig. 828), the cells of two fila- 

 ments develop on their adjoining sides a small tubular process ; 

 these ultimately meet, and adhere, and the intervening septum 

 existing at the point of contact becoming absorbed, the two 

 cells freely communicate together. The contents of the cells 

 then contract into a mass, and ultimately combine together, 

 cither by the passage of the contents of one cell into tlie other, 

 or by the mixture of the contents of the two cells in the tu- 

 bular process between them. Under either circumstance, the 

 mixture of the contents of the two cells results in the formation 

 of a resting spore, which ultimately germinates into an individual 

 resembling its ])arents. 



2. Imprcgnatiuii of naked spores or germ-corpuscles by ciliated 

 spermatozoids. — Tliis mode of fecundation has been observed 

 in some of the Chlorosporca;, in Volvociuca), and in Mclano- 



