310 XXII. REPRODUCTION OF ALG.E. 



branch opens. The tubular investing sacs close under the crown 

 elongate a little, by which the outer layers of membrane of the 

 sacs are torn at this spot. As the result of this we see the sheath 

 which previously had been incrusted with lime right up to the 

 crown, now become free from lime just under the crown. Simul- 

 taneously with their elongation the investing sacs have also 

 separated from one another, and thus clefts arise which reach 

 right into the interior, to the apex of the oosphere. By elongation 

 and lateral separation of the investing sacs, therefore, a short 

 " neck " has arisen under the crown, rendering fertilisation of the 

 oosphere possible. If, indeed, we examine in the early morning 

 the oogonia found in the immediate neighbourhood of the last 

 opened antheridia, we shall find in and upon the clefts of the neck 

 numerous spermatozoids clinging to them. They are arrested 

 here by a jelly-like substance. The fertilised egg, or oospore, 

 becomes surrounded by a strong colourless membrane, and the 

 inner wall of the sheathing sacs bounding upon this commences 

 after some time to thicken and become brown. In spite of the 

 incrustation of lime, these relations can be made recognisable by 

 treating the oogonium with hydrochloric acid. 



