No. I.] THE EMBRYOLOGY OF THE UN 10 NI DAE. 57 



of small cells (Schmidt, No. 31 and Schierholz, No. 30), which 

 are destined to form the pericardium, nephridia, and perhaps 

 other mesoblastic structures. At no stage of their embryonic 

 history are these cells contractile. 



To return to the myocites. Schierholz has distinguished 

 six on each side ; but as of these, two pairs belong to the 

 primary mesoblast, there are but four pairs of myocytes constant 

 in position. The two most prominent pairs are shown in Figs. 

 75 and ^6 ; the relative position of these will be better under- 

 stood after reference to Fig. 73, in which the planes of the 

 sections are indicated. Another pair passes from the oral 

 plate to the entodermic sac (Fig. 69) ; and, sometimes at least, 

 there is still another pair, partly indicated in Figs. 73 and 74, 

 attached on the one hand just beneath the aperture of the 

 thread-gland and on the other just in front of the oral plate. 

 A very conspicuous strand is that shown in Fig. 79, running 

 from the entodermic sac to the shell near the postero-dorsal 

 border of the adductor muscle. 



What is the function of these cells } Schierholz assigns to 

 them an important mechanical part in development. Thus by 

 their steady and slow contraction they produce invaginations, 

 e.g., the lateral pits, or the larval mantle ; again they shift the 

 relative position of the parts. These ideas are on a par with 

 his suggestion that the oral plate is produced by the pressure 

 of the overlying polar bodies (which as a matter of fact lie 

 slightly in front of this plate). One cannot deny the originality 

 of this view of the mechanics of development ; neither can one 

 accept it. The theory of unequal growth as the cause of in- 

 vagination and kindred phenomena in development was never 

 better illustrated than in the Unionidae. As a matter of fact, 

 wherever in the embryo invaginations occur, clusters of active 

 nuclei are to be observed {cf., e.g., the oral plate. Fig. 75, or 

 the lateral pits. Fig. 95). It is true that myocytes are attached 

 to the region of the oral plate ; it is however quite common for 

 the larval oesophagus to possess such attachments {cf. Hatschek, 

 on Teredo). As for his theory of the lateral pits, the "Strang- 

 zellen" in that neighborhood are not myocytes, but derivatives 

 of the primary mesoblasts. 



