NEPHRODIUM GAMETOPHYTE 175 



3. The depressed apex of the prothallus, which is occupied, 

 not by a single wedge-shaped cell, as is the case in early stages 

 of development, but by a closely aggregated series of marginal 

 cells, with thin cell-walls, and every appearance of recent and, 

 repeated cell-divisions. 



4. The antheridia, which are hemispherical outgrowths, 

 situated chiefly on the posterior and lateral portions of the 

 under side of the prothallus. 



5. The archegonia, which are situated on the cushion near to 

 the organic apex of the prothallus ; the multicellular neck of the 

 archegoniun projects from the surface of the prothallus as an 

 elongated cylindrical structure. 



Under the low power select one mature antheridium, and, 

 without moving the slide, adjust the higher power so as to 

 observe the structure of the same antheridium in detail. It will 

 then be seen that it consists of 



a. A wail, composed of a single layer of narrow cells ; this 

 completely surrounds 



b. The spermatocytes, or mother-cells of the spermatozoids, 

 which are small, and not very numerous. 



Other antheridia may be found which have already burst the 

 outer wall : in these the contents of the mother-cells may perhaps 

 be seen escaping from the ruptured antheridium as spiral 

 spermatozoids, endowed with active movements. If a prepara- 

 tion showing motile spermatozoids be treated with a weak 

 solution of iodine, the movements will cease with the death of 

 the spermatozoids, which will assume a brown staining, while 

 the cilia attached to the anterior ends of them will then be 

 clearly seen. 



Select under the low power one mature archegonium, and 

 then observe it in detail under the higher power. If the neck 

 be vertical, which would under the circumstances be the natural 

 position, since the prothallus was mounted with the lower surface 

 uppermost, there will then be seen, on focusing down upon it, 

 four rows of cells composing the wall of the neck, and surround- 

 ing one cell, the canal-cell. 



IV. Harden some prothalli with alcohol, or with picric acid 

 and then with alcohol. The preparations described below may 



