66 FECUNDATION ; MOTILE ISOGAMETES. 



Both Oltmanns and Berthold agree in the opinion that Ectocarpus 

 siliculosus may be either mono3cious or dioecious, for they observed 

 individuals whose gametes would not conjugate with each other, but 

 only with those of another individual. As is well known, the gametes 

 are generally borne in the so-called plurilocular sporangia. The details 

 in the process of nuclear and cell-division in the development of both 

 gametes and asexual swarm-spores have not, as yet, been thoroughly 

 studied. The gametes (Fig. 19, A) are pear-shaped cells with a chro- 

 matophore, nucleus, a reddish brown eye-spot, and two cilia inserted 

 laterally. The cilia are of unequal length, the longer extending for- 

 ward and the shorter backward. 



The conjugation of the gametes can be most readily followed in a 

 hanging drop, into which both male and female gametes are intro- 

 duced, when the whole process may be observed with the aid of the 

 highest magnifying powers. The female gametes, as a rule, first come 

 to rest, and about each one numerous spermatozoids assemble. If the 

 female gamete comes to rest at the edge of the drop, the male cells 

 cluster about it, attaching themselves apparently by the anterior cilium, 

 giving the familiar picture figured by Berthold (Fig. 19, A). But 

 should the female gamete attach itself to some particle hanging in the 

 arched surface of the drop, this cell then appears as a circular disk 

 surrounded by a wreath of male cells radially disposed. Shortly a 

 male gamete (in exceptional cases two), having attached itself to the 

 female by means of the anterior cilium, approaches the latter appar- 

 ently by the sudden contraction of the same and unites with it, while 

 the remaining male gametes withdraw (Fig. 19, B, C). In a few 

 minutes cytoplasmic union is complete, and within about ten hours 

 after copulation both nuclei have fused (Fig. 19, E, F, G). The 

 chloroplasts do not unite, a fact which is contrary to the peculiar 

 phenomenon described by Overton for Spirogyra (see page 69). 



The sexual process in Ulothrix, Hydrodictyon, and Ectocarpus 

 may be considered as fairly typical of the lower algae in which fecun- 

 dation consists in the fusion of motile isogametes. In this, probably 

 the simplest and most primitive sexual process, as in the higher plants, 

 it will be seen that fecundation consists in the fusion of the sexual 

 nuclei together with the cytoplasm of the gametes, but the fusion of 

 the nuclei must be regarded as of prime importance. 



