62 University of California Publications in Zoology [Vol. 20 



Division of the Neuromotor Apparatus and Ectoplasmic 

 Structures 



The onset of division is marked by certain nuclear changes that 

 will be discussed later. The first evidences of it in the grosser struc- 

 tures of the organism are found in the change from a bell-shape to 

 a spherical contour in the body as a whole (pi. 7, figs. 24, 31). With 

 this change the nucleus migrates anteriorly until it comes to lie 

 immediately below the centroblepharoplast (fig. 31). No trace of a 

 connecting rhizoplast has been found. This change in the form of 

 the bod}^ results in a spreading apart of the myonemes and fibrils of 

 the ectoplasmic layers, with an apparent enlargement of the alveoli, 

 so that these structures are more easily studied in this stage than 

 in the more usual but contracted form of the normal vegetative 

 trophozoite. 



The next step in the division process is found in the partition 

 into two parts of the lobes at the base of the tubular portion of the 

 neuromotor system, the centroblepharoplast. With this occurs a sep- 

 aration of the entire ectoplasmic laj^er into two parts, with a small, 

 spindle-shaped portion of endoplasm appearing in the chasm thus 

 made (pi. 7, fig. 30; pi. 8, fig. 35). At about the same time or slightly 

 earlier, the operculum-like, transparent cap with the cup-sliaped 

 depression which it covered, disappears, and the tip of the tubular 

 neuromotor apparatus reaches quite to or near the outer surface of 

 the anterior end of the body. 



The splitting or division of the tubular part of the neuromotor 

 apparatus or centroblepharoplast proceeds from the base anteriorly 

 to the tip (pi. 8, figs. 33, 34), and with the splitting each half draws 

 together its parted edges until they meet and each moiety foi-ms a 

 new tube. The attachment to each other at the tips may persist for 

 some time, as such stages are more common than are the intermediate 

 stages shown in plate 8, figure 33. As the two halves of the tube 

 separate, strands of darkly staining material are found joining the 

 two inner surfaces near their bases (pi. 8, fig. 34). This is the 

 paradesmose which functions in the formation of the spindle, and is 

 apparently drawn out from the material of the centroblepharoplast 

 itself. With the final separation of the tips of the new daughter 

 tubes the paradesmose remains . as the only connecting link between 

 the two structures (pi. 8, fig. 36). 



