No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. fi, 



commences, as might be expected, along the line of the suture- 

 cells, and is at first most active near the thread-gland (Fig. 82). 

 It is accompanied by very pronounced changes of form on the 

 part of the invaginating cells. The greater mass of the proto- 

 plasm migrates to the inner end of the cells (Fig. 85); the 

 nucleus accompanies it — a very usual appearance in large 

 invaginating cells ; the cells then roll up and in towards the 

 shell, which thus comes to be lined by two layers on each side 

 (Figs. Zy and 90) : first, the protoplasmic layer already spoken 

 of (which is very intimately attached to the shell) ; and, second, 

 the cells of the invaginated mantle. Beginning, as I have said, 

 in front, the invagination passes backwards, in proportion as 

 the oral plate travels towards the ventral plate, which, on its 

 part, moves forward to meet the former. The rest of the 

 mantle rudimetit is thus divided into lateral halves and invagi- 

 nates mider the same appearances, carrying with it the median 

 ventral and oral plates, which have now met and occupy the 

 whole of the posterior region. 



The vacuolation of the larval mantle cells begins in quite an 

 early stage — about the stage of Fig. 79. Flemming has fig- 

 ured and described the appearance of these cells at this time. 

 The vacuolation, which is not at first very marked, soon 

 becomes more and more exaggerated (Fig. 85). Indeed, it 

 seems as though there was an active effort on the part of the 

 cells in question, with a given amount of protoplasm, to cover 

 the greatest space possible. The transition from the compact 

 columnar cells of Figs. ^6, yy, and y^, to the flat, much- 

 vacuolated cells of Figs. 84 and 93 is most striking. 



The suture-cells are well seen in Fig. 90 and in section in Figs. 

 85, 86, and 87. They are long, spindle-shaped, deeply staining 

 cells, with rod-like nuclei ; in cross section they are wedge- 

 shaped (PL VI, Fig. 85). 



During the invagination of the mantle, the thread-gland has 

 shifted its position backwards along the line of the suture-cells, 

 and now lies just in front of the oral plate. Although the 

 displacement of the thread-gland takes place along the line of 

 the suture-cells, yet I should hesitate to attribute any active 

 share to these cells. They seem rather to be the preformed 



