ON THE DEVELOPMENT OF LINGULA ANATINA. 45 



85.) which run toward the body proper, two or three of them 

 uniting in their course (Cf. PI. VIII., Fig. 131., r. m.). The 

 presence of such muscle fibres has been observed by Kowalevsky 

 ('83) in the lame of Cislella (p. 64.). 



The inner epithelium of the mantle margin facing the mantle 

 cavity is composed of columnar cells. It changes, however, into 

 the gland zone, (PI. VIL, Fig. 98, gl. ell.) at a little distance 

 from the margin and just proximal to the pigment zone. In 

 toto preparations the gland zone is seen to be composed of large 

 polygonal gland cells (PI. VIL, Fig. 85, gl. ell.), as Simeoth 

 figures ('97 Fig. 5.). The formation of these cells is begun at 

 the postero-lateral corner of the mantle and gradually spreads 

 forward (PI. VI., Fig. 85, gl. ell.) until they cover the entire 

 circumference of the mantle. The gland cell is filled with secre- 

 tion granules which stain intensely with hematoxylin or are 

 colored a light violet with carmalum. It is quite invisible in 

 fresh materials. The nuclei are pushed aside by the granules 

 (PI. VIL, Fig. 98, gl. ell.). The gland cells, it may be noted, 

 give rise to the interesting gland-ridge (Drüsenwall) of the adult. 

 Proximal to the thickened margin the inner epithelium remains 

 very thin and it becomes almost impossible to determine the line 

 separating the inner and outer layers of the mantle. In this 

 region the nuclei are infrequent and scattered here and there 

 (PL VI., Fig. 85.). 



As to the outer epithelium of the mantle there is nothing 

 worth mentioning except that it increases slightly in thickness 

 at the mantle margin. 



Setse are first seen at the close of the 7 p. c. stage along 

 the entire margin of the mantle. At the 8 p. c. stage nearly all 

 the set« are still so short that they hardly reach the shell margin, 



