ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 31 



rest and from the walls of the cavity, forming the endoderm; the 

 remainder form two lateral masses adhering to the outer wall 

 (ectoderm), constituting the mesoderm, with somewhat the characters 

 of a " germinal streak." The endoderm and mesoderm re-unite to act 

 as a nutritive vitellus. Stage 4. Cylindrical Embryo. — The cells of 

 the zone increase greatly in height, and form most of the sides of the 

 embryo, which is now almost globular. A cap consisting of radiating 

 cells, is formed from the ectoderm beneath the epidermis at the 

 aboral pole. At the posterior side of the oral extremity an invagina- 

 tion is formed, and forces its way into the interior, becoming a very 

 large organ, the sac (stomach of the author's former memoir) ; in 

 front a rounded organ of doubtful origin appears, at first unattached ; 

 this is the " glandular organ." Stage 5. Oblique Embryo. — The form 

 is elongated from the oral to the aboral end ; by the superior growth 

 of the posterior cells of the ciliated zone, as compared with the 

 front ones, the hind face becomes much longer than the front face ; 

 the latter is, therefore, thrown to the side, and the shape of the 

 embryo becomes oblique and asymmetrical. The sac, now very large, 

 still opens at the oral pole, while the glandular organ faces towards 

 the oblique part of the hinder pole, and henceforth forms a part of 

 the ciliated zone. At the aboral pole a lateral invagination (formerly 

 termed the rudiment of the sucker) forms the pallial cavity, and extends 

 as far as the ciliated zone. The fr( mt part of the central region of the 

 oral face consists of large flat cells, the peripheral portion of small 

 cylindrical cells ; the space between is pushed inwards, to form the 

 ciliated aperture ; a group of cells which radiate from its anterior part 

 carries the plumule. The glandular organ unites with those cells 

 and with the ciliated aperture to form the pyriform organ. From the 

 lower part of the ciliated aperture two small lobes become marked off. 

 The vitelline mass now becomes degenerated into a nimiber of 

 globules scattered in the general cavity. 



Metamorphosis: — Stage 1. Half-open Larva. — The sac is evagi- 

 nated through the orifice by which it was invaginated, and forms a 

 quadrangular base for the fixation of the embryo. The peripheral 

 part of the oral face is forced into the interior below the ciliated 

 zone, which closes over it all, with the exception of the two small 

 lobes. The ciliated zone also covers the whole aboral aspect, which 

 now consists of long cells and spreads out and becomes thin in pro- 

 portion as the pallial cavity increases in size. 



Stage 2. Umbrella-shaped Larva. — The zone, fixed at the central 

 part of the oral face, is turned back, thus causing the disappearance 

 of the pallial cavity ; and the surface of the aboral face grows out- 

 wards, and extends an umbrella-like sheet of substance over the 

 inverted zone ; this curves over towards the base and then comes into 

 contact with the quadrangular plate of fixation, and forms the 

 interior wall of a cap whose exterior is formed by the aboral sus- 

 pension. The central part of the oral face becomes a short connect- 

 ing tube. 



Stage 3. Lozenge-shaped Larva. — It is the sac which gives the 

 embryo this shape when seen from the aboral end, owing to two 



