10 DE. J. P. GEMMILL ON THE DEVELOPMENT OF 



apertures in each of several interradii. The same Starfish, when spawning on 

 April 2nd, took up a somewhat different position, viz., on one side of the tank right at 

 the surface, with the uppermost arms unattached, bent over, and separated so as to 

 expose the interradial angles. The ova were emitted chiefly from these interradii, but 

 also from one or two others, sometimes singly, sometimes two or three together, at 

 short intervals, for two and a half hours. Some years ago I observed a specimen of 

 Solaster papposa in the act of spawning. The process was almost exactly identical 

 with that which has been last described. 



At the time of shedding, the eggs may be compressed in any direction, but they very 

 quickly assume their proper form, that of an " oblate spheroid " (p. 7). Being rather 

 lighter than sea-water, they rise gently to the surface and remain there when un- 

 disturbed. As was indicated previously, they float with the equator of the spheroid 

 horizontal. The upper pole is of a rich orange-colour, and has in its centre the small 

 area free from fatty yolk. The lower pole is paler in colour and of distinctly 

 greater specific gravity, so that the eo^g rights itself almost at once, should its balance 

 be disturbed. 



The vitelline membrane is closely applied to the surface of the egg all round. In 

 the eggs shed on the first two occasions it was quite thin, transparent, and smooth on 

 the surface, but in those which were shed last there was an additional outer covering, 

 somewhat frayed on the surface, and with minute granular debris adherent. The 

 difference is probably caused by the addition to the true egg-membrane of a mucous 

 coat formed within the ovary itself. 



Two other specimens of ^S*. endeca were kept in the same tank as the one which 

 spawned, and fortunately one of these proved to be a ripe male. On March oOth, 

 while ova were being shed by the female, the male was observed resting in an 

 ordinary position on the side of the tank about two feet distant, and sending out 

 thin streams of sperm from five or six interradii. On this occasion, practically all the 

 ova were fertilized, although in the end only a few of them developed quite normally 

 beyond the gastrula stage. On the next occasion of spawning (April 2ud) the emission 

 of sperm by the male was not obvious. All the eggs were, however, fertilized, and a 

 very large proportion of them underwent normal development. 



The eggs of Solaster are much too large and opaque to allow the process of fertili- 

 zation to be watched under the microscope. It can readily be seen, however, that they 

 attract the sperm strongly as soon as they are shed, and therefore a considerable time 

 before the maturation-changes are completed. No doubt the disc at the upper pole 

 and the stalk leading from it down to the centre of the egg serve as a track of 

 entrance and guidance to the spermatozoon, which would find difficulty in boring 

 a way through the densely crowded mass of fatty yolk-granules present every- 

 where else. 



