12 HARLEY N. GOULD 



obliquely across the visceral sac. The gonad is minute. In one 

 of the smallest specimens 39 nuclei of active germ cells were 

 counted in serial sections. Allowing for those which may have 

 been overlooked, there were at least no more than 50 germ cells 

 present. At this time the gonad is triangular in cross section 

 (fig. 1) with one side parallel to the intestine. The angle oppo- 

 site this side is drawn out gradually into a strand of cells which 

 runs forward and to the right, until it meets the extreme right 

 posterior angle of the mantle cavity, where it ends. This strand 

 is the anlage of the goniduct (fig. 2, d, and fig. 62). 



The gonad is surrounded by a thin layer of connective tissue. 

 The germ cells are contiguous to the connective tissue sheath, 

 and are not all of the same nature. In the youngest specimens 

 which it has been possible to obtain two types of germ cells may 

 already be distinguished. Since the presence of dimorphic germ 

 cells in so early a stage of development is significant in the light 

 of the "indifferent germinal epithelium" theories, the cells will 

 be described in some detail at this point. 



Types of germ cells 



Throughout the life of Crepidula plana, up to the assumption 

 of the adult female condition, two types of germ cells, which we 

 may call 'type A' and 'type B', are at all times present in the 

 gonad and may be distinguished from each other in the following 

 ways: 1) The type A cells are larger than the type B cells and 

 have a different nuclear pattern. 2) The type A cells occur only 

 in the periphery of the gonad, while the type B cells occur both 

 in the wall and in the lumen. 3) The two types of cell have dif- 

 ferent times of maximum activity. 4) Two types of division 

 figures are present which correspond to the two different kinds 

 of germ cells. 



1. In the type A series the nucleus is very transparent and 

 the chromatin is disposed either in deeply staining masses or in 

 strands which suggest a persistent spireme (figs. 1, 2, 3, 4, 7, 

 etc.). Good fixation reveals the fact that these chromatic 

 elements are of a duplex nature ; they are composed of two short 

 rods or strands lying side by side. The type B cells (figs. 1, 2, 



