FREE-SWIMMING SPOROSACS OF THE HYDROID GENUS DICORYNE. 263 



Development of the Sporosacs. 

 A. The Female Sporosacs (PI. VII). 



The oocytes are differentiated and grow in the ectoderm of a blastostyle in the 

 positions which the sporosacs will subsequently occupy ; there is no migration of 

 oocytes from endoderm to ectoderm, nor does there appear to be any movement of 

 oocytes in the ectoderm. The differentiation and early phases of growth of an 

 oocyte precede the formation of the sporosac which is to contain it. Oocytes of 

 apparently very different ages are present in the reproductive zone of a single blasto- 

 style, and there seems to be no definite rule as to the period of growth or size of 

 oocyte governing the first appearance of the sporosac. For instance, small oocytes 

 7—10 ft in diameter have been observed within distinct ectodermal swellings, but 

 two others, each about 18 ft in diameter, lie in ectoderm the surface of which 

 is not elevated. 



The first stage of formation of the sporosac is due directly to the growth in 

 volume of the oocyte. As the bulk of the latter increases the overlying ectoderm is 

 raised into a dome-shaped mound, and the ectoderm cells surrounding the oocyte 

 become considerably elongated. At this stage (fig. 2) the sporosac-elevation is 

 entirely ectodermal, no changes having taken place in the neighbouring endoderm 

 and mesoglcea. 



Further increase in the size of the oocyte soon involves the formation of a more 

 definite ovoid outgrowth (fig. 3). The ectoderm cells covering the rapidly growing 

 oocyte multiply and form a single layer of moderately regular cells ; at the same 

 time the subjacent endoderm increases and the mesoglcea becomes arched outwards 

 and diminishes in thickness to a mere film. The base of the sporosac thus becomes 

 occupied by a mass of endoderm — the spadix — connected with the endoderm of the 

 blastostyle by a narrow neck. The cells of the spadix are arranged radially and 

 with considerable regularity, and there is a small axial cavity (frequently filled with 

 granular matter as indicated in the figure) continuous with the ccelenteron of the 

 blastostyle. The base of the oocyte applied to the endoderm lies obliquely with 

 regard to the axis of the gonophore, and this obliquity is still further increased as 

 growth proceeds, the spadix eventually being pushed to one side of the oocyte. 



In the older sporosac, shown in fig. 4, the spadix has a large central cavity and 

 its cells contain brownish granules which apparently increase in number with the 

 age of the sporosac. Some time prior to the stage figured there appears, immediately 

 proximal to the sporosac proper, a constriction which is of great importance in the 

 subsequent history of the sporosac. For convenience of description we shall refer 

 to the region proximal to the constriction as the sporosac-stalk, to the distal region 

 as the sporosac, and to the connection between the two at the zone of constriction 

 as the sporosac-neck. In the sporosac-stalk the ectoderm retains its original character 

 and resembles that of the blastostyle, being composed of relatively large cells with 



