64 LILLIE. [Vol. X. 



path of displacement, and serve to separate the halves of the 

 larval mantle as well. Schierholz has figured a muscle-cell 

 connecting the thread-gland with the oral plate, and attributes 

 to it the function of causing this displacement. 



The ventral plate goes through some interesting changes (to 

 which I have already referred) about this time. Figs. 83 to 

 89, representing sections in various planes through the stage 

 of Fig. 82, illustrate the description. The ventral plate grows 

 past the posterior margin of the larval mantle (which represents 

 the anterior end of the blastopore) and towards the oral plate 

 (Figs. 83 and 84), which is at the same time moving backwards 

 through the larval mantle. At this time the oval plate can be 

 recognized only as a cluster of deeply staining nuclei (Fig. 84), 

 all traces of its previous pit-like condition being obliterated by 

 the great expansion of the cells of the larval mantle. A tongue 

 of cells of somewhat similar appearance to the ventral plate at 

 the opposite end of the embryo might lead to the belief that 

 similar processes of development were responsible for the two 

 structures ; and hence that the appearance of free, forward 

 growth of the ventral plate was illusory. But that this is not 

 the case is proved by horizontal sections (Figs. 86 and ^j). 

 These show that at the anterior end of the embryo {i.e.y 

 directly opposite the ventral plate) there is a protrusion of the 

 cells of the larval mantle without the shell, caused of course 

 by the violent contraction of the adductor muscle on the addi- 

 tion of kiUing reagents. There is nothing of this sort to be 

 seen at the posterior end. It is the section of the protruding 

 cells which is seen in sagittal section at the anterior end. {Cf. 

 lines of section in Figs. 84 and 87.) I have hence been forced 

 to conclude that there is an actual growth forward of the 

 ventral plate above the cells of the larval mantle. The oral 

 plate has been moving backwards through the larval mantle at 

 the same time. During the invagination of the mantle the two 

 structures meet and unite (Fig. 88). 



The mantle cells have rolled away to the side ; and hence the 

 whole of the median portion of the glochidium, from the oral 

 plate to the posterior angle of the shell, is formed from the 

 ventral plate (Fig. 93). According to the united testimony of 



