264 DR J. H. ASHWORTH AND DR JAMES RITCHIE ON THE 



correspondingly large oval nuclei. The ectoderm cells of the sporosac are smaller 

 and of a different type ; they have undergone rapid division, resulting in the formation 

 of a thin layer of uniform squarish or oblong cells with small, spherical, deeply 

 staining nuclei. Occasional small oval nematocysts are present in the ectoderm of 

 the sporosac and the cells are finally ciliated. A thin continuous cuticle envelops 

 the whole ectoderm both proximal and distal to the constriction. In the sporosac- 

 stalk the mesogioea is of moderate thickness, but at the constriction it rapidly 

 diminishes in amount and in the sporosac is very thin. At an early stage in the 

 differentiation of the neck-region of the sporosac an outgrowth — consisting of 

 ectoderm and endoderm — appears at the base of the sporosac. This outgrowth 

 undergoes rapid development and becomes a finger-like process — the future tentacle 

 of the sporosac — with ectoderm like that of the sporosac and with solid endoderm in 

 a single column of cells. The tentacle is confined between the ectoderm and cuticle 

 of the sporosac, its tip being directed distally. 



With the further development of the sporosac the constriction at the neck 

 becomes more and more pronounced. The ectoderm of the sporosac and that of the 

 stalk no longer maintain continuity, with the result that in its final stage the neck 

 consists only of a narrow tube of mesoglcea with a slender central core of vacuolated 

 endoderm (fig. 4). 



During the later phases of growth of the sporosac and oocyte a narrow peripheral 

 zone of the latter becomes differentiated ; it is more finely granular than the 

 remainder of the yolky cytoplasm and reacts differently to stain (p. 278). Several 

 of the larger oocytes examined in sections are enveloped by a thin pellicle about 

 1 /x in thickness — the vitelline membrane. 



By the time the oocyte has attained a diameter of about 80-90 /a the tentacle 

 of the sporosac is fully developed, the neck has been reduced to its minimum, and the 

 sporosac is ready to break away from the blastostyle and become free-swimming. 

 The process of release appears to be purely mechanical, and to be due to the action 

 of currents in the surrounding medium playing upon the relatively large bulk of 

 the sporosac and breaking its slender attachments. At any rate, no histological 

 change could be observed in sporosac-stalks from which the sporosacs had recently 

 escaped. The escape is evidently due simply to the rupture of the loose cuticle in 

 the neighbourhood of the neck and then of the thin neck itself. In a few cases 

 where the cuticle had failed to rupture, the sporosacs have remained attached to their 

 stalks, and their respective oocytes have undergone degeneration. 



On the sporosac becoming free the loose cuticle which covers it is usually soon 

 discarded. The tentacle, now released from its confined position, becomes extended 

 so as to lie more or less in line with the long axis of the sporosac, in the position 

 shown in fig. 5. The numerous cilia of the general ectoderm and of the tentacle 

 come into play and the sporosac swims, at the same time rotating on its own axis, 

 probably with the tentacle directed anteriorly, though the observations on the 



