86 FECUNDATION; HETEROGAMETES. 



becomes attached to the surface on which it may be resting. We consider it 

 as certain that the flight of the supernumerary antherozoids marks the moment 

 of actual fertilization, and it seems only possible to interpret the events outside 

 the egg as the results of an excretion from it of some substance which not only 

 exerts on the surrounding antherozoids a negative chemotactic but also a 

 directly injurious effect, for a number of dead sperms may be seen around the 

 fertilized egg. Possibly the bead-like filaments which partly stain like muci- 

 lage, are directly concerned in the process. 



The facts observed by Farmer and Williams have been given some- 

 what in detail, because they are suggestive of various interesting 

 problems, especially those pertaining to chemotaxis between sexual 

 cells, a province of physiology well worthy of careful investigation, 

 and one which will undoubtedly yield fruitful results. 



It may be noted that, in the attachment of the spermatozoids to the 

 egg by means of one cilium, and in the sudden withdrawal of the super- 

 numerary sperms as if startled, a certain resemblance exists between 

 Halidrys and Ectocarpus (see p. 66), although these phenomena 

 are less marked in the latter. 



In the case of normal healthy products, fecundation occurs within a 

 few minutes after the addition of the male cells. The fecundated eggs 

 form a membrane around themselves at once, and behave in a very 

 different manner from those into which no spermatozoids have pene- 

 trated. For example, if the sea-water be gradually drawn off from a 

 mixture of fecundated and non-fecundated eggs, the latter flatten out, 

 their cytoplasm loses its coherence and becomes distributed in all 

 directions, while the former show only local protuberances and burst 

 only at one point. 



The passage of the sperm-nucleus through the cytoplasm and its 

 fusion with the nucleus of the egg can be followed with anything like 

 accuracy only in thin and properly stained sections. According to 

 Strasburger ('97), the egg of Fucus platycarpus at the time of fecun- 

 dation is globular and provided with only a plasma membrane. The 

 alveoli of its cytoplasm, together with the included chromatophores, 

 are radially disposed about the centrally placed nucleus (Fig. 30, A), 

 an arrangement which seems to facilitate the movement of the sperm 

 to the egg-nucleus. The passage of the sperm through the cytoplasm 

 and its union with the nucleus of the egg take place rapidly, for both 

 Strasburger and Farmer agree that ten minutes after the addition of 

 the spermatozoids to the water containing the eggs the sexual nuclei 

 have united. Strasburger is inclined to the view that the larger por- 

 tion of the cytoplasm of the spermatozoid on entering the egg unites 



