REPRODUCTION. 607 



process. When, as in this case, the two coalescing cells 

 are externally quite similar, the term conjugation is applied 

 to the process ; the coalescing cells are simply termed gametes, 

 or, when as in this case they are ciliated and motile, piano- 

 gametes; and the spore resulting from the coalescence is 

 termed a zygospore. The cell in which the gametes are 

 produced is termed z. gametangiwn. 



In some of the Siphoned, such as Acetabularia and Botry- 

 dium, the asexually produced spores are ordinarily resting- 

 spores, though an adventitious production of uniciliate zoo- 

 spores may take place in Botrydium. On germination, the 

 protoplasm of the resting-spore undergoes division to form a 

 number of ciliated cells which are set free. In Acetabularia 

 these cells are sexual, conjugating in pairs; they are piano- 

 gametes, and the cell producing them a gametangium. This 

 is true also in the case of Botrydium provided that the 

 germinating resting-spore is young. When the resting-spore 

 is old, the cells to which it gives rise on germination, as 

 Rostafinski and Woronin point out, do not conjugate, but 

 develope independently, though they are externally similar 

 to the planogametes. We have, in fact, a case of partheno- 

 genesis. 



On reviewing the foregoing facts, we see that the entirely 

 sexual planogametes of Acetabularia can be traced back, 

 through Botrydium and Ulothrix, to the entirely asexual 

 microzoospores of Haematococcus. We may conclude that all 

 gametes are derivatives of the spore, and that all sexual 

 reproductive organs are derivatives of the sporangium; in 

 a word, that sexual has arisen out of asexual reproduction. 

 To this point we shall again refer subsequently. 



Up to this point we have had to deal with gametes which 

 are externally similar, but in our further discussion of the 

 subject we come now to gametes which are more or less 

 dissimilar either in the size or form, or in the part which they 

 take in the sexual process. These external differences are 

 indications of a physiological difference which constitutes 

 sex. Without entering at present into a discussion of the 

 nature of sex, which we reserve for a future occasion, it may be 



