i" PHYLUM AND CLASS PORIFERA 



right side). These gemnmles are formed in the substance of the 

 Sponge towards the end of the year; they are set free by the 

 decay of the part of the parent sponge in which they are de- 

 veloped, and fall to the bottom. In spring the contained mass 

 of protoplasmic matter reaches the exterior through an aperture- 

 the micropyh in the wall of the gemmule, and develops into the 

 adult form. 



All Sponges multiply by a sexual process by means of 

 male cells, or sperms, and female cells, or ova. These arc 

 developed from certain of the amoeboid wandering cells of the 

 mesoderm, which take up a special position, usually immediately 

 below the collared cells of the endoderm. Ova and sperms are 

 developed in the same Sponge, but rarely at the same time. The 

 amoeboid cell destined to form sperms divides into a number of 

 small cells, giving rise to a rounded mass of sperms. The latter, 

 when mature, have oval or pear-shaped heads and a long tapering 

 appendage or tail. Each amoeboid cell destined to form an ovum 

 enlarges, and eventually assumes a spherical form. After a sperm 

 has penetrated into its interior and effected impregnation, the ovum 

 usually becomes enclosed in a brood-capsule formed for it by certain 

 neighbouring cells, and in this situation, still enclosed in the parent 

 Sponge, it undergoes the earlier stages of its development. 



In Sycon the course of the development is as follows. Imme- 

 diately after impregnation the ovum divides into two cells ; each 

 of these again divides into two, the plane of the second division 

 being at right angles to that of the first. A vertical radial fissure 

 then appears, dividing each of these four cells into two ; so that the 

 embryo (Fig. 82, b, c) now consists of eight cells, which are of a 

 pyramidal shape, and arranged in one layer in a radiating manner, 

 in such a way as to form a flat cone with a central aperture. The 

 apices of the eight pyramidal cells are next separated off as a 

 ring of eight small cells from the rest of the cells, which 

 remain as eight larger cells. The eight small cells are the endoderm 

 cells, the eight larger are the ectoderm cells. The cells are arranged 

 so as to form the wall of a sphere the Uastula (Fig. 82, d) 

 with a central cavity, the ectoderm cells being on one side of the 

 sphere and the endoderm cells on the other. The endoderm cells soon 

 increase greatly in number by further division, and remain clear ; 

 the ectoderm cells divide more slowly, and become granular. The 

 clear cells become elongated, and flagella are developed at their 

 outer ends (Fig. 82, e). The granular cells become pushed 

 inwards so as to be partially enclosed by the clear cells, the space 

 (segmentation cavity or Nastoccele) in the interior of the blastula he- 

 coming greatly reduced. In this stage of development termed the 

 amphiblastula (Fig. 82, e) the embryo Sycon escapes from the 

 enclosing capsule into the flagellate canal and reaches the exterior. 

 It is now an oval body consisting of a mass of cells, of whi"h 

 VOL I. I 



