PLATE 1 



EXPLANATION OF FIGURES 



1 to 4 and 6 Cross sections through parts of a gonad of immature speci- 

 mens of Crepidula plana. X 681. 



1 From a very small specimen, less than 1 mm. entire shell length. The 

 primordial male {B) and female {A) cells are distinguishable and their differences 

 are as marked as at any later time in the sexual cycle. A number of spermato- 

 gonial cells have been formed and deposited in the lumen, but have later de- 

 generated forming irregular, homogeneously staining bodies. 



2 From a specimen about 1.5 mm. length. Although slightly larger than the 

 individual from which figure 1 was taken, this specimen has not developed any 

 spermatogonia. It has only primordial male (B) and female (A) cells and 

 follicle cells (/). The space marked 'd' shows where the rudimentary goniduct 

 joins the gonad. In many small specimens it has been observed that just at 

 this point the wall is very thin, consisting only of the connective tissue sheath 

 which surrounds both gonad and duct. 



3 Specimen about the same size as that from which figure 2 was drawn. 

 The gonad is further developed, however; a number of spermatogonia have 

 been formed and lie in the lumen. This figure is drawn at the same magnifica- 

 tion as figures 1 and 2; a comparison shows great differences in cell size in the 

 different specimens. This may be partly accounted for by degree of activity in 

 cell division. 



4 Section through periphery of a gonad in which spermatogonia! multipli- 

 cation and a few divisions of primordial egg cells were going on. The specimen 

 was 7 mm. long, with small outgrowth in the region where the penis will form. 



5 Section through part of the gonad of a post-larval Crepidula convexa less 

 than 1 mm. long. The stage of development represented is comparable to that 

 of figure 1 in C. plana. As in the latter species, there are two types of primordial 

 germ cells present. 



6 Section through part of a nearly adult testis. All stages of spermato- 

 genesis up to and including spermatids were found in this gonad, though no 

 spermatids appear in this figure. Differentiation between the eupyrene sper- 

 matocyte {spcl. at top of figure) and apyrene spermatoblast (spb) is clear. A 

 group of late primary spermatocytes (spcl at lower part of figure) and one sper- 

 matosome {sps); besides several spermatogonia (spg) are also shown. There 

 is no very definite germinal layer around the periphery of the gonad. Length 

 of specimen, 11 mm. 



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