HARDWICKE'S SCIENCE-GOSSIP. 



157 



are suspended two or three very refractile granules, 

 or nucleoli ; the protoplasm in the centre of the cell, 

 surrounding the nucleus, is more or less granular, 

 but towards the margin perfectly clear. 



ThQ mesoderm, which succeeds, is of a very different 

 nature : the great mass of it consists of a clear trans- 



cells pass into others of a simpler form, by losing 

 their branching processes, and becoming fusiform. 

 The fusiform cells so produced are of considerable 

 length, and lie in parallel bundles concentrically 

 round the edges of the gastral ostia ; and since 

 they have the property of shortening in the long 



ec 



c.S 



Fig. 96.— Two free stages in the development of Sycandra raphanus. (Copied 



Jrom F. E. Sckulze.) A, Amfhisblastula stage ; B, a later stage, after the 



ciliated cells have commenced to become invaginated : cs, segmentation 



cavity ; ec, granular cells which form the ectoderm ; en, ciliated cells, which 



become invaginated to form the endoderm. 



Fig. 97. — Fixed gastrula stage of Sycandra 

 raphanus. (Copiedfrotn F. E. Schulze.) The 

 figure shows the amoeboid ectoderm cells {ec') 

 derived from the granular cells of the earlier 

 stage, and the columnar endoderm cells, lining 

 the gastrula cavity, derived from the ciliated 

 cells of the earlier stage ; the larva is fixed by 

 the amoeboid .cells on the side on which the 

 original mouth opening is situated. 



parent jelly-like material, which 

 does not stain with carmine 

 or other colouring reagents ; 

 dispersed through this jelly-like 

 matrix are numerous cells of 

 branching form. Each contains 

 in the middle a long oval nu- 

 cleus, with spherical nucleolus, 

 and is surrounded by finely 

 granular protoplasm, from 

 which are produced a number 

 of fine irregularly branched 

 filaments, which anastomose 

 with similar filaments from 

 similar adjacent cells. In 

 general terms, therefore, this 

 tissue may be said to consist of 

 a network of protoplasmic cells 

 immersed in a quantity of clear 

 indifferent jelly ; the jelly, in 

 all probability, being derived 

 from the contained cells by 

 metamorphosis. Altogether the 

 tissue most closely resembles 

 the jelly-like tissue of the disc 

 of the medusa, or jelly-fish, and 



is also related to the embryonic connective tissue of 

 the higher animals. 



The matrix of the mesoderm serves as a medium 

 for two other forms of cells, in addition to the stellate 

 or branched ones ; in some parts of it, particularly 

 in the neighbourhood of the gastral ostia, the stellate 



Fig. 98. — The young of 

 (Copied from F. E. 

 tremity ; os, osculum 

 terminal osculum and 



Sycandra raphanus shortly ; fter the development of the spicules. 

 Schulze.) ,A, View from the side ; B, view from the free ex- 

 ; ec, ectoderm ; en, endoderm, composed of ciliated cells. The 

 lateral pores are represented as oval white spaces. 



direction, and broadening in the transverse direction, 

 under the influence of a stimulus, they serve as 

 muscular sphincters to the ostia, closing them when 

 irritated and opening them again on relapsing into 

 their normal state. The third kind of cell found in 

 this tissue is probably the least differentiated, and is 



