380 RICHARD EVANS. 



whenever they are situated near a surface. Whether the surface 

 in question is int«ernal or external seenas to be immaterial. 



To sum up; the characters of the cells with granular 

 nuclei are as follows : — an irregular or flattened form ; clear 

 cytoplasm, which may contain a few enclosures; and a fairly 

 large nucleus with a thin nuclear membrane, which encloses 

 a great number of small granules placed at the nodes of a fine 

 reticulum. 



(/3) The Cells with Vesicular Nuclei. — These cells 

 for the most part occupy the posterior moiety of the inner 

 mass. They are seldom found in the anterior region, 

 between the layer of flattened cells which line the larval 

 cavity and the flagellated layer. In the youngest larvae they 

 are not separated from the larval cavity situated anterior to 

 them, nor from the flagellated layer at the posterior end, but in 

 older ones a layer of cells with granular nuclei is developed in 

 both of these positions. Consequently the cells with vesicular 

 nuclei, mingled with cells of other kinds, become restricted to 

 the internal part of the solid posterior end of the inner mass. 

 These cells are by far the largest in the whole larva, and con- 

 trast with the cells which possess granular nuclei in having a 

 perfectly definite outline as well as an oval or circular form, 

 at least in the younger larvae ; in older specimens they seem 

 to lose to some extent their regularity of outline and compact- 

 ness of form. Another point of contrast between these two 

 classes of cells is that the cells with granular nuclei contain but 

 a few enclosures, while those with vesicular nuclei are always 

 possessed of one or more '' nutritive vacuoles ^' and several 

 ''yolk bodies.^' The number of these enclosures found in the 

 individual cells depends on the stage of development of the 

 larva. In young larvae the cells often contain as many as 

 three or four nutritive vacuoles and numerous yolk bodies, 

 while in older larvae they almost invariably contain only one 

 nutritive vacuole and but a few yolk bodies. These cells, 

 ill common with the two classes of cells already described, 

 possess some of the small refringent granules mentioned in 

 describing the flnjrellated cells. 



