124 Object and Function of the Egg-stalk. 



degree. Compare (Plate III, Fig. 8) the ^^^% of Biorhiza 

 aptera taken from the ovary, with another laid in 

 January, and taken out of the bud in the beginning of 

 April. What causes the striking increase in the circum- 

 ference ? Evidently half of the ^%% is filled with fluid. 

 The embryo lies at the posterior pole, and scarcely 

 occupies half the egg-chamber ; in front of it lies a sac 

 containing fluid, which does not enter into the egg-stalk, 

 but stops short at its commencement. The embryo is 

 surrounded by a very delicate membrane, floating, so 

 to speak, in this fluid. The egg-stalk takes part in the 

 expansion, particularly at its club-like end, and is also 

 filled with fluid. What can be the use of this con- 

 trivance? We have seen that the club-shaped end of 

 the egg-stalk is the last to enter into the portion of the 

 plant prepared to receive it, and remains close to the 

 surface. It is separated, as a rule, from the outside air 

 by a single thin bud-scale only, and consequently this 

 part of the egg-stalk is accessible to the physical 

 influences of the atmosphere, and admits of gaseous 

 interchange. The fluid in the club-like end, enclosed 

 by a very delicate membrane, can thus absorb oxygen, 

 and as the egg-stalk is merely an extension of the ^^% 

 cavity, oxygen can be conveyed to the embryo in this 

 way. Therefore, as I understand it, the egg-stalk has the 

 function of a respiratory organ. 



In support of this theory I would adduce the following 

 facts. The embryo of Cynipidae eggs requires a supply 

 of oxygen at a very early stage of development. A long 

 time before it comes to perfection, it is the seat of 

 continual movements. The Biorhiza aptera q^^ (PI. Ill, 



