882 ECOLOGY 



morpha, there are small conjugating gametes and large parthenogenetic 

 gametes. The auxospores of diatoms may develop vegetatively, may 

 form asexual spores, or may conjugate sexually. In Ectocarpus there 

 are transitions between sporangia and gametangia, the same structures 

 producing either zoospores or isogamous or heterogamous gametes, 

 thus suggesting the possible origin of sex as well as of sexuality. In all 

 of these cases the exact determinative factors remain to be discovered, 

 although it has been suggested from their small size in comparison with 

 zoospores and parthenogenetic gametes that conjugating gametes 

 represent poorly nourished spores. 



Artificial parthenogenesis. The most important experimental evi- 

 dence concerning parthenogenesis is derived from animals, and in view 

 of its great significance, it must be cited here. Parthenogenesis is ob- 

 served somewhat frequently in a number of animals, such as bees, wasps, 

 and plant lice ; in the latter it occurs especially at high temperatures or 

 when the host plant is very watery. It has been demonstrated that the 

 eggs of the sea urchin (Arbacia) develop into larvae in the absence of 

 sperms, if they are placed in somewhat concentrated solutions of mag- 

 nesium chlorid and sea water. Comparable results were obtained with 

 other salts, and all were at first referred to the increased osmotic pressure 

 occasioned by their presence. Hence it was suggested that the stimulus 

 necessary for egg development is the extraction of water. Later ex- 

 periments have demonstrated that artificial parthenogenesis can be 

 brought about in many other ways than by exposing eggs to increased 

 osmotic pressure, and it is becoming evident that the explanation of the 

 process is by no means simple; a feature of recent experiments is the 

 emphasis that has been placed upon chemical factors. 



Experiments with similar results have been made upon the eggs of other echino- 

 derms than the sea urchin (e.g. those of the starfish, Asterias), and also those of 

 certain annelids (as Chaetopterus and Polynoe) and mollusks (as Sottia). In the 

 starfish, eggs develop parthenogenetically when they are exposed for several hours 

 to temperatures below 7 C. It was found some time ago that the eggs of Chae- 

 topterus develop parthenogenetically by the addition to the medium of potassium 

 ions in an amount too small to produce an osmotic effect, and more recently various 

 acids and alkalis have been seen to act similarly. Treatment with a fatty acid 

 (as acetic acid) before placing in a concentrated solution greatly stimulates develop- 

 ment, because it causes the formation of a membrane, just as when a sperm fuses 

 with the egg. The mechanical agitation of eggs sometimes causes their partheno- 

 genetic development. Indeed it would seem that almost any disturbance may serve 

 to stimulate the development of certain eggs. Probably a large factor in the case 

 is the permeability of the egg to the substances it needs for its development, and it 



