112 Transactions of the Royal Canadian Institute, [vol. x. 



The embryo is composed of two portions, an inner, the oncosphere, and 

 an outer, the mantle or so-called ectodern, well supplied with cilia. The 

 movements of the latter can be seen even when the embryo is within 

 the shell, especially if a little pressure be applied to the cover-glass. In 

 this case they vibrate so vigorously that the whole embryo is driven 

 to the larger end of the egg, and numerous, supposedly vitelline granules, 

 are kept continually in motion, and, at the same time, arranged in two 

 groups, one close to the mantle and, to all appearances, among the bases 

 of the cilia, and the other in the smaller posterior end of the egg. The 

 shell is provided at its anterior end with a well-defined operculum, the 

 raising of which, evidently due to the pressure from within, permits the 

 escape of the embryo. This, however, is a somewhat difhcult matter on 

 account of the size of the latter, as can be seen from Fig. 33. As fast as 

 the cilia are freed they proceed to vibrate strongly in the surrounding 

 saline solution, and as soon as the embryo has escaped from the shell it 

 swims away quickly, taking either straight courses or moving about 

 erratically in irregular curves. It was also noticed that the cilia are all 

 directed posteriorly from what might be called the apex of the body, both 

 within and without the shell, — posteriorly, since this apex is anterior, not 

 only from its direction during motion, but from its being situated at the 

 end of the oncosphere opposite to that which accommodates the hooks. 

 While the mantle is comparatively constant in size, its diameter being 

 about 45;ii, the oncosphere varies from 30 to 35 /x in length. Practically 

 no structure was observed in the substance of the mantle itself. The 

 oncosphere, on the other hand, shows the usual three pairs of hooks, a 

 pair of flame-cells and a few spherical bodies of doubtful significance 

 (Fig. 32). The movements of the hooks and of the body of the oncos- 

 phere are quite typical. They take place even when the embryo is yet 

 within the shell, but, as has been verified in many preparations, only 

 when the whole egg has been stimulated by pressure, in which case they 

 are quite irregular and necessarily considerably restricted. They are 

 perhaps a little freer when the embryo, including the ciliated mantle 

 in situ, is liberated. At rest the three pairs of hooks are arranged, as 

 shown in Fig. 32, in the form of a tetrahedron, the apex of which is 

 situated at the center of the oncosphere while the base is directed pos- 

 teriorly. From this position the peripheral ends of the hooks approach 

 each other until they are quite close together, while the central ends 

 diverge towards the bounding membrane of the oncosphere. This 

 causes a slight retraction of the tip of the latter. Then follows a com- 

 paratively vigorous separation of the hooks, to the extent that their 

 outer ends are about 180° apart while the inner are close together. At 

 the same time the individual hook protrudes from the surface of the 



