No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 5 1 



five cells persist in their characteristic position, and without 

 any alteration of nuclear structure, until the glochidium is fully 

 formed. 



Fig. 74 is a part of Fig. 73 much more highly magnified. 

 The nucleus occupies the inner end of the gland, and is iden- 

 tical in appearance with those between whose cells the gland 

 opens. The lumen penetrates to within a short distance of 

 the nucleus, and then disappears. Rows of granules may be 

 seen radiating from the nucleus to the inner end of the lumen. 

 The position of the gland, the character of its nucleus, and 

 the presence of five nuclei only round its opening in place 

 of the six original cells, make certain its derivation from the 

 central cell of Fig. 71. The terminal nucleus persists for 

 some time longer (Fig. 79), but gradually dwindles and dis- 

 appears. During its existence it no doubt controls the growth 

 in length of the gland, but after its disappearance the further 

 growth seems to be supported by the five large cells around 

 its opening. 



Briefly, the further history of the thread-gland is as follows : 

 It grows backwards beneath the hinge line until it reaches the 

 posterior end of the body; then turns down, and passes on 

 the right of the entodermic sac to the large cells of the larval 

 mantle, which it enters at the angle of the shell. The apical 

 nucleus has by this time disappeared. The gland continues 

 its growth through the cells of the larval mantle, until it 

 reaches its anterior end, when it turns dorsad to its opening 

 beneath the anterior end of the hinge line. This is the extent 

 of the gland in the stage represented in Fig. 82 (PI. VI). In 

 later stages a still greater length is attained. After the invagi- 

 nation of the mantle the gland takes two or three turns around 

 the adductor muscle in the right valve of the shell. 



How is the thread formed >. It is hardly conceivable that 

 such a long (10 to 15 mm. Forcl) and strong structure should 

 be formed by the activity of a single cell. Nor is it, in spite 

 of the fact that the gland arises from a single cell. The thread 

 must be thought of, not so much as a secretion into the lumen 

 of the cell, as an actual metamorphosis of the substance of 

 the cell (PI. VI, Figs. 85, 91, etc). When this has begun (in 



