408 M.N. Lieberkuhn on the Development of 



usually somewhat more pointed at one end, like a hen's egg. 

 The smaller forms are not half so large, just as similar variations 

 exist in the size of the gemmules. In most specimens, without 

 the microscope, a transparent hemispherical space may be di- 

 stinguished in the anterior, and a shining white one at the pos- 

 terior part of the body ; the distinction of anterior and posterior 

 being based upon their position when swimming, which takes 

 place at about the same rate as in Trachelius ovum. They swim 

 in all directions : sometimes at the surface of the water, next 

 towards the bottom, gliding along this, and then rising towards 

 the surface again ; sometimes in straight lines, at others forming 

 a circle. When two of them meet, they often swim for some 

 minutes around each other, subsequently going apart ; frequently 

 they remain motionless for a time, and then start off again. If 

 touched when at rest, they swim aw^ay. They remained in this 

 state for one or two days, when they went to the bottom of the 

 vessel, where they adhered and began to decay. In but few 

 instances, notwithstanding numerous experiments, have I suc- 

 ceeded in inducing their development. After the above time, 

 the substance of the bodies becomes expanded into a delicate 

 layer, in which a structureless mass with the fine siliceous needles 

 is soon all that can be distinguished : the experiments succeeded 

 when spring-water was applied. On the 20th day I remarked 

 that the spots formed by the spores had become larger. Ex- 

 amination showed the presence of the constituents of young 

 sponges, viz. moveable cells, smaller and larger needles, and some 

 germ -granules. The movements are effected by cilia regularly 

 spread over the entire body. They are of about the same 

 length as those of the Turbellaria, but more slender. But what 

 distinguishes them at once from the ciliary apparatus of all 

 known Infusoria, and from that of the Turbellaria, so accurately 

 examined by Schultze, is a kind of epithelial layer upon which 

 they are situated. This consists of a single layer of spherical 

 cells, about jj^-^ inch in diameter. The cells are not so crowded 

 as to flatten each other, but they are mostly in contact. I have 

 not as yet detected a nucleus or nucleolus in them, but they 

 usually contain some highly refractive granules. 



On watching a swarm-spore under the microscope, part of the 

 epithelial layer is not unfrequently seen to separate from some 

 part of the body, — eight or ten connected cells often becoming 

 detached and set in motion in the liquid by their cilia. Each 

 cell has a single cilium, and never more than one. In a few not 

 perfectly fresh swarm-spores, the surface was divided into several 

 circular and irregular spaces, which under a low magnifying 

 power appeared like large cells, but under a high power be- 

 came resolved into groups of the above- described small cells. 



