I 



BANCROFT: OVOGENESIS IN DISTAPLIA OCCIDENTALIS. 83 



Fig 24, cl. tst.) to the condition where the test cell is entirely within 

 the latter, but still connected by a narrow strand with the follicle 

 (Plate 3, Fig. 24; Plate 4, Pig. 27, cl. tst/), and finally to that where 

 it is entirely disconnected from the follicle, and the chorion passes 

 between the two (Fig. 27). 



The way in which the test cells develop here is essentially that 

 given by Morgan ('90) and Floderus (pp. 234, 235), but the evidence in 

 the three cases differs slightly. Morgan has differentiated the bounda- 

 ries between the adjacent follicle cells, which neither Floderus nor 

 I have found, and has seen the cytoplasm of the forming test cell 

 extending between the follicle cells quite to the peripheral membrane 

 of the latter. Floderus finds both the follicle cell and the developing 

 test cell within a clear space, which he considers hyaline cytoplasm. 

 I find that in favorable instances the cytoplasm of the follicle and test 

 cells stains, and the two are seen to be in connection. When, however, 

 the cell boundary between them can be detected (Fig. 25), the test cell 

 cannot be seen to extend quite to the periphery of the follicle, but 

 lies on its central surface. In this respect Distaplia differs from the 

 forms studied by Morgan, for the first step in the formation of the test 

 cell in Distaplia seems to be a displacement of the whole follicle cell 

 towards the inner surface, and only subsequently does its separation 

 take place. 



Julin (p. 123), in describing the ovogenesis of Styelopsis, says that 

 there is an almost simultaneous mitotic division of all the follicle 

 cells, and when the products of this division come to rest, they are 

 arranged in two layers, the inner of which consists of test cells. 

 Caullery ('94, p. 600) agrees with Julin in deriving the test cells 

 from the follicle by mitosis. I have never seen any mitotic figures 

 in the follicle or test cells of Distaplia occidental is, but do not doubt 

 that they occur. It is likely, however, that in this species the process 

 is not accomplished as Julin describes it, for after the mitosis there 

 must be a differentiation taking place, by means of which the nuclei 

 of the prospective test cells become larger and more spherical tlian the 

 remaining follicle nuclei. 



Shortly after the origin of most of the test cells, the chorion is 

 secreted between these cells and the follicle (Plate 4, Fig. 27); even 

 this, however, does not destroy the cytoplasmic connections of all the 

 test cells, and occasionally the union can be seen at much later stages. 

 With the formation of the chorion, the differentiation of the primitive 

 follicle into secondary follicle and test cells appears to be complete, 



