DESCKIPTIOX OF PLATE yil—contlnuerl. 
4b. — Sperm-cells from same, older, the nucleus having become more dis- 
tinct. 
ic. — Sperm-cells from same, still larger, having become more elongated, 
nucleus not having yet begun to be concentrated, a, Nucleus; 
/3, cell-contents. 
4d — Sperm-cells from same, still more elongated and compressed ; nucleus 
contracted to a spot, a, Nucleus, which ultimately becomes 
caput of zoosperm; /3, part of cell, which becomes the tail of 
zoosperm. 
4:6. — A single sperm-cell, which has nearly attained the condition of a 
young zoosperm. a, Nucleus, elongating into caput; /3, tail. 
Limn6Bus dagnalis. 
5a. — A group of the youngest cells from dichogamic gland of Z. stagnalis, 
each cell measuring "OOOS". 
hb. — A group of older cells from same, each cell measuring '0005". 
he. — Sperm-cells from same, which have grown larger, and have just begun 
to elongate, measuring -0011" in length and •0005" in breadth. 
5fi?. — Sperm-cell from same, which has just begun to present the appear- 
ance of a young zoosperm. a, Caput ; /3, tail. 
Helix aspersa. 
6a. — A group of the youngest sperm-cells from dichogamic gland of H. 
aspersa, each individual cell measuring 0004:". 
65. — A group of older cells from same, measuring about '0007". 
Gc. — Cells from same, already beginning to elongate, the nucleus not 
having, as yet, begun to change, a, The nucleus. 
6d. — A group of sperm-cells from same, showing the progressive concen- 
tration of the nucleus, a, Nucleus. 
6e. — A sperm-cell from same, elongating, the nucleus still showing as a dot, 
a, Caput ; /3, tail. 
6/. — A sperm -cell, still more elongated, the nucleus also now beginning to 
elongate, a, Caput ; /3, tail. 
