No. 3.] STUDIES ON THE HETERONEMERTINI. 391 



the various groupings of these vacuoles within the cell as corre- 

 sponding to the different physiological states of the latter. 

 Rarely does the spongioplasm preponderate quantitatively 

 throughout the cell, though this is the rule for the ganglion 

 cells of the commissures of the oesophageal nerves. 



To recapitulate : a fine-grained layer of spongioplasm forms 

 the peripheral boundary of the cell, and a second similar layer 

 envelops the nucleus. The remainder of the cytoplasm is 

 usually coarsely vacuolar, especially distally, these hyaloplasmic 

 vacuoles being of unequal size and without appreciable arrange- 

 ment into concentric layers around the nucleus. Now the fine- 

 grained layers around the periphery of the cell and encircling 

 the nucleus I would consider alveolar layer's, in the sense of 

 Biitschli ('94), their microsomes thereby representing the nodal 

 points of an alveolar meshwork, and the unstaining, hyaloplas- 

 mic spaces between these supposed nodal points the more fluid 

 contents of the meshes (" Waben "). Whether similar alveolar 

 layers also bound the larger vacuoles I have been unable to 

 determine, even after careful investigation. According to Biit- 

 schli's conclusions, these larger vacuoles would not correspond 

 to individual meshes, since they exceed the size of a primitive 

 mesh ("Wabe," which, according to his studies, rarely exceeds 

 I /A in diameter, although occasionally it may measure 8/i). On 

 the whole, however, the cytoplasmic structure of cells II and III, 

 in both Linens and Cerebratulus, and of IV in the latter (I being 

 too small for investigation), gives the impression of a honey- 

 combed meshwork, as described by Biitschli for the cytoplasm 

 of especially vacuolar Protozoa, such as Actmosphaeriimi} Even 

 the presence of the two alveolar layers described by me would 

 alone render it probable that this is the structure of these 

 ganglion cells, since Biitschli has shown pretty clearly that the 

 presence of such alveolar layers is only to be explained on the 

 presupposition of a honeycombed meshwork. 



^ Schaudinn, in a number of papers upon Rhizopoda published in the last three 

 years, has confirmed Biitschli's results on the structure of the protoplasm of the 

 Protozoa. I would express the view reservedly that the honeycombed meshwork 

 probably represents the structure of the less differentiated protoplasms, and is the 

 more primitive structure; but that this structure may become radically changed in 

 more differentiated cells. 



