STRUCTURE AND FORM OF THE MATURE SPERMATOZOID. 53 
in his earlier investigations on Vaucheria he was mistaken in describ- 
ing the dissolution of the //autschicht of the sporangium and the for- 
mation of a new //autschicht around each spore (112, p. 188). In the 
ease of Gdogonium, also according to Strasburger, the //autschicht 
of the sporangium goes to form the //autschicht of the swarm-spore. 
Strasburger says: ‘* Die Hautschicht des Sporangiums liefert auch 
hier thiitsichlich die Hautschicht der Schwiirmspore.” 
In studying the living pollen tubes in sugar solutions, considerable 
search has been made for evidence bearing on this point. In no case, 
however, has a definite membrane connected with the stalk cell been 
found inclosing the spermatozoids, which could be considered as the 
wall of the mother cell. When the spermatozoids pull apart, how- 
ever, an appearance is sometimes observed which might suggest the 
presence of a mother cell wall. If mature pollen tubes, in which cilia 
motion has not begun, are placed in sugar solution, the cilia begin to 
vibrate and the spermatozoids gradually pull apart and round up, as 
described elsewhere in this paper. When the cilia first begin their 
motion the surrounding protoplasm seems to hold together and spring 
back and forth by the beating of the cilia, as if bordered by a definite 
membrane. When the spermatocoids round up they occupy less space 
than when they are attached and quiescent. In certain tubes the pro- 
toplasm surrounding the spermatozoids holds together tenaciously 
when the spermatozoids begin motion, strongly suggesting the pres- 
ence of a mother cell wall. When the spermatozoid strikes against it 
or when hit by the cilia, the protoplasmic mass does not break up, but 
shows elasticity, springing in and out with the impinging of the 
cilia, etc. This is seen to some extent whenever spermatozoids are 
observed starting their motion, but it is seldom very noticeable. 
Usually the protoplasm soon breaks up and the spermatozoids swim 
about unobstructed. While these observations suggest the presence 
of a mother cell wall, the writer believes that it must be interpreted 
in another way, as it is certain from a study of prepared sections that 
no distinct wall from that of the mother cell is formed around the 
spermatozoids. It would seem that the protoplasmic structure is 
not easily broken up and hangs together tenaciously in some cases. 
Again, the plasma membrane of the pollen tube, which is a single cell, 
surrounds the entire prothallial apparatus, and it is probably this 
membrane which gives the spermatozoids some difficulty in breaking 
through into the general protoplasmic contents of the pollen tube. 
The plasma membrane of the pollen-tube cell is not so easily differen- 
tiated as that of the cells of the prothallial apparatus, but is usually 
clearly distinguishable, and would probably form an obstruction to 
any object like a spermatozoid entering it. Considering the structure 
and phenomena presented, the writer has been led to the conclusion 
that the description of the structure given in his preliminary paper is 
