SMALLWOOD : MATURATION OF HAMINP^A SOLITARIA. 293 



responding bodies near the inner or deep centrosome. The interzonal 

 filaments are faint and irregular. The centrosomes have likewise become 

 faint, and the centrioles, though distinguishable, no longer take a very 

 dai'k stain. 



At the next stage shown (Plate 12, Fig. 83) the centrosome of the 

 outer pole has disappeared, and the one at the inner pole is less distinct 

 than in the preceding stage. The medullary and cortical layers of the 

 sphere within the egg are still present, though not sharply defined. 

 Each chromosome now becomes hollow ; its walls are at first thick and 

 . take a heavy stain ; but as the vesicle grows, its walls become thinner. 

 These nuclear vesicles lie in a cluster around the fading centrosome 

 (Fig. 84). The centrosome may be recognized until tlie vesicles begin 

 to fuse together to form the female pronucleus (^protil. 9 ) when the 

 boundary of the centroplasra becomes wrinkled and shrunken, and no 

 centriole can be distinguislied. 



In some eggs, not reproduced, persistent rays pass between the vesicles 

 formed from the chromosomes, and though they radiate from a common 

 point, there is no recognizable body at that point, the centrosome having 

 completely disappeared. As the vesicles begin to fuse into one large vesi- 

 cle, small particles of chromatin make their appearance within the vesicles. 

 The female pronucleus (pron/. 9 ), resulting from this fusion of vesicles, is 

 irregular in outline when first formed (Plate 8, Fig. 53 ; Plate 12, Fig. 85), 

 and larger than the male pronucleus (pronJ. ^ ). During the fusion of the 

 vesicles to form the female prouucleus the astral rays break down and dis- 

 appear in the cytoplasm, although a few niay be pi'esent as late as the stao-e 

 shown in Figure 85. The female pronucleus is usually surrounded by a 

 layer of fine granules, the outer limit of which merges imperceptibly 

 into the surrounding cytoplasm. Tliis is the substance of the medullary 

 layer of the sphere, which has now spread itself around the female pro- 

 nucleus. This condition is similar to that observed by Conkliu (:01, 

 p. 283) in Crepidula, but is unlike the condition described by Lillie 

 ('97, p. 245 ff".) forUnio. However, this sphere-substance does not follow 

 as definite a course of changes in Haminea as it does in Crepidula. 



A few unusual conditions have been found in Haminea. Two of these 

 are represented in Figures 58, 59 (Plate 9). In Figure 58 the outer 

 limiting boundary of the parent centroplasmic mass is present, and ex- 

 tends about halfway around the new achromatic figure. Tiie line is 

 apparently continuous, and yet the new astral rays pass thmugh it. 

 The gradual disappearance of the sphere-substance and the encroachment 

 of the cytoplasm is thus well illustrated. The new cortical layer about 



