LAG EN I DI ALES 907 



due to the intake of water, since vacuoles are common, or to the 

 accumulation of host protoplasm. 



In species of Olpidiopsis parasitic in water molds, the development 

 and cytology of which are well known from the work of several inves- 

 tigators (particularly Barrett, 1912b; Diehl, 1935; McLarty, 1941b; and 

 Shanor, 1939b, 1940), the very young thallus is uninucleate, slightly 

 granular, and surrounded by a delicate membrane (McLarty, op. cit.). 

 As it enlarges, according to Barrett, the refractive apparently fatty 

 granules increase in number and size and a cellulose wall is secreted by 

 the thallus, which has by now assumed a spherical or ellipsoidal shape. 

 The contents then become more dense and are augmented by many 

 small granules and an increased number of refractive fat bodies. The 

 host cytoplasm at this stage forms a dense layer around the parasite, 

 and protoplasmic streamers, between which are large vacuoles, are 

 produced. These protoplasmic strands radiate from the region of the 

 parasite to the peripheral cytoplasmic layer of the hypha of the host. 

 This radiate disposition of the host contents is less extensive as devel- 

 opment proceeds. The thallus of the parasite then becomes vacuolate, 

 the fat bodies within it disappear, and its protoplasm assumes a densely 

 granular texture. It may rest in this stage for several weeks, or its 

 contents may cleave into zoospores at once. 



The cytological investigations of Olpidiopsis ve.xans by Barrett ( 1 912b) 

 (see also McLarty, 1941b) show that the young uninucleate vacuolated 

 thallus (Fig. 74 A, p. 908), after about doubling its original size, becomes 

 binucleate (Fig. 74 B). The nuclei thus formed are large and have a well- 

 defined nucleolus. Division is mitotic, the spindles being intranuclear 

 (Fig. 74 C-D). As the rather evenly contoured thallus increases in size 

 it is continually more vacuolate, and the nuclei are augmented in number 

 by simultaneous divisions. In an advanced, but as yet unwalled, stage 

 the protoplasm and nuclei line the periphery of a large irregular central 

 vacuole (Fig. 74 E). At about this time a distinct granulation of the 

 outside margin of the thallus is detected, which probably marks the 

 initial stages in wall formation. There then ensues a gradual inward 

 growth of the protoplasm, a decrease of vacuolization with continued 

 rapid nuclear division, and an increase in size. A well-defined wall has 

 now been formed. After the last division of the nuclei (Fig. 74 F) the 



