SMALLWOOD : MATURATION OF HAMINEA SOLITARIA. 279 



We may now inquire into the origin of the centroplasm. MacFarland 

 ('97) explains the origin of the inner sphere (the centroplasm in Hami- 

 nea) in Diaulula as resulting from a concentration of the substance 

 surrounding the Centralkorn. In regard to this he says (p. 251) " Dem 

 entspreohend ist audi die dichtere Substanz noch nicht zu annahernder 

 Kugelform an den beiden Polen concentrirt, sondern besitzt etwa die 

 Form flacher, planconvexer Linsen. Etwas weiter gehende Entfarbung 

 ahnlicher Figuren bringt das Centralkorn im Centrum der Polsubstanz 

 zur Anschauung, sowie einige Fibrillen, welche die beiden Flatten ver- 

 binden." Lillie (: 01, p. 239) says in regard to Unio, " The entire inner 

 sphere [the centroplasm in Haminea] is the product of a single centro- 

 some." He further says that the granules which mark the outer limit 

 of this inner sphere are derived from the centrosome. In Crepidula 

 (Conklin, : Ol), similarly to Unio, the medullary zone (the centroplasm 

 in Haminea) is surrounded at first by four contiguous bodies which have 

 themselves been derived from a single body, the centrosome. These 

 four bodies fuse into a ring which marks the outer limit of the medullary 

 zone. Here we have, then, three rather distinct methods of formation 

 for the centroplasm, all of which lead to the same general result ; that is, 

 a centrosome differentiated into centroplasm and centriole. 



The centroplasm in Haminea begins to appear while the eggs are in 

 the ovotestis. In Figure 16 (Plate 3) there can be distinguished im- 

 mediately surrounding a central granule a very narrow area into which 

 the rays do not penetrate. As soon as this area is distinguishable, 

 the centrosome may be considered to consist of two parts, — centro- 

 plasm and centriole. As the centroplasm (" area ") increases in extent, 

 the centriole becomes irregular in shape. This irregularity continues 

 during the growth of the centroplasm, which in Haminea seems to 

 be derived entirely from the centrosome, as stated for other animals 

 by MacFarland, Lillie, and Conklin. Here, however, it arises by a pro- 

 cess different from that described by either of these authors. The cen- 

 triole itself is as large in Figures 19, 21 (Plate 4) as are the centroplasm 

 and centriole together in Figure 16 (Plate 3) ; that is, the process in 

 Haminea involves an increase in the size of the centriole coincident 

 with irregularities of outline. 



Cortical Layer. The cortical layer (st. ctx.), a brief description of which 

 has been given in a previous section (p. 267), shows no differentiation 

 in eggs from the ovotestis, both primary and secondary rays extending 

 to the centrosome ; but in eggs that have been laid, the cortical layer 

 exhibits two regions or zones. The inner zone takes a very light plasma 



