728 LECTURE XLI. 



from mutually confronting cells of the two filaments, protuberances have grown out, 

 as at a and d, which come into contact with one another; at the points of contact 

 their cell-walls then dissolve, and a joint canal is thus formed between the 

 two confronting cells, as seen in Fig. 410 A at a. Meanwhile the thin lining of 

 protoplasm which clothes the cell-wall, and in which the band of chlorophyll lies, 

 contracts so as to form a spherical mass, a process only possible by the expulsion 

 of water. Now, however, comes in a distinction between the two mutually con- 

 fronting protoplasmic bodies : the one remains stationary, whereas the other, as 

 if attracted by the former, puts out a process into the canal, which comes into 

 contact with the former, and as soon as this contact is accomplished, the whole of the 

 movable protoplasmic body glides over to the other, the process resembling the 

 contact and fusion of two oil-drops floating on water (cf. Fig. 410 A, a). 

 The union is complete. After its accomplishment no trace is perceived of the 

 fact that the body 5 (in Fig. 410 ^4) has arisen from two; even the two spiral bands 

 are said to join ends and become one. The zygote thus produced now surrounds 

 itself with a membrane composed of several shells, and after the fertilised filaments 

 of the Spirogyra have sunk to the bottom of the water the zygotes contained in 

 them remain dormant until the next spring ; they then germinate in the manner repre- 

 sented in Fig. 24 (p. 33), and, ascending to the surface of the water, again develope 

 into long filaments divided into chambers by means of transverse divisions. 



In Spirogyra, more distinctly than in the examples previously considered, the 

 fact comes forth that fertilisation consists essentially in the fusion of the contents 

 of two cells. In contrast to the slow movement, which is here executed by only 

 one of the two sexual cells (which we may designate male) a great number of Algae 

 are now known whose sexual cells swim actively about in water like ordinary 

 asexual swarm-spores, and then, when they come into mutual proximity, attract 

 each other, come into contact, and then fuse together. To distinguish these from 

 non-sexual swarm-spores, such sexual swarm-spores are called Gametes, and the 

 product of their union, which usually moreover has to undergo a period of rest, the 

 Zygote. I will describe these processes in connection with certain Algae of the family 

 Volvocinese, in which this form of fertilisation was first observed in 1869 by 

 Pringsheim. 



Pandorina Moruni, Fig. 411 /, is one of the commonest of the Volvocinese. 

 The sixteen cells forming an individual or coenobium are packed closely together and 

 surrounded by a thin gelatinous envelope, from which the long cilia project. The 

 asexual multiplication is accomplished by each of the sixteen cells repeatedly breaking 

 up into sixteen smaller cells, which group themselves into a coenobium in exactly the 

 same manner as will be described below for Eudorina. The sixteen daughter- 

 ccenobia (//) become free by the dissolution of the gelatinous envelope of the parent, 

 and each of them, again invested with a gelatinous envelope, grows up to the 

 original size of the parent family. The sexual reproduction also is commenced in 

 exactly the same way. The gelatinous envelopes of the young families deliquesce, 

 and the individual cells are thus set free, and each swarms by itself independently 

 (///). These free swarmers diff'er much in size ; they are rounded and green at the 

 posterior end, and pointed, hyaline, and provided with a red corpuscle at the anterior 

 end which also supports the two cilia. During the swarming of these bodies, some 



