COPULATION OF GAMETES. ECTOCARPUS. 65 



COPULATION OF GAMETES. 



The gametes of Ulothrix zonata are rounded or oval cells, bearing 

 two cilia at the anterior end (Fig. 17, E). Each contains a nucleus, 

 a red eye-spot, situated about midway between the ends of the cell 

 near the surface, and a chromatophore. According to Strasburger 

 the cilia are developed under the influence of the nucleus and from the 

 anterior, colorless portion or mouth-piece, which consists mostly of 

 kinoplasm. In his later investigation of the subject of swarm cells, 

 Strasburger (1900) finds that the cilia arise from a local kinoplasmic 

 thickening of the plasma membrane at the anterior end. As already 

 mentioned in a preceding paragraph (p. 47), he regards this thicken- 

 ing as the homolog of the blepharoplast of the Archegoniates. In the 

 swarm-spores of Hydrodictyon, Timberlake finds a small body at the 

 base of the cilia, which, in some cases at least, was not a part of the 

 plasma membrane. 



The gametes copulate in pairs immediately after they escape from 

 the gametangium (Fig. 17, F, G). It is probable that they may be 

 brought together, or at least held together after coming in contact, by 

 means of a chemotactic stimulus. The stigmatic or eye-spots do not 

 unite, but remain separate and independent in the young zygote (Fig. 

 17, H). There is no doubt of a nuclear fusion, but how soon this 

 takes place after conjugation is not known, so far as the author is aware. 



In Hydrodictyon the gametes are small, oval in shape, biciliate, 

 containing one nucleus and, according to Klebs, two pulsating vacuoles. 

 They conjugate in pairs immediately on escaping from the gametan- 

 gium, but I have observed that conjugation may sometimes take place 

 within the mother-cell. If, however, copulation does not follow soon 

 after the gametes are set free, they become incapable of uniting, come 

 to rest and disorganize. Whether this is a rule was not determined. 



ECTOCARPUS. 



Among the isogamous Phceophycece the sexual process is doubtless 

 best known in Ectocarpus siliculosus Lyngb. from the investigations 

 of Berthold ('Si), which have been confirmed and extended by Oltmanns 

 ('99) . Ectocarpus is of especial interest in this respect, since it repre- 

 sents a transition from isogamy to heterogamy. In fact, there is in the 

 brown algae, as well as in phylogenetic series of other Thallophyta, 

 every transition from the type of gametes found in Ectocarpus to that 

 of Fucus. The gametes, although nearly or quite the same size and 

 appearing morphologically alike, are physiologically different, and we 

 may, with much propriety, speak of egg-cells and spermatozoids. 



