Aug. 24, 1883.1 



♦ KNOAA^LEDGE ♦ 



123 



trustworthy guide to the very numerous celestial objects of 

 beauty and interest which are well within the reach of a 

 by no means costly instrument. As a means of perennial 

 amusement and gratification, scarcely anything aflbrds such 

 a return for money laid out as a telescope ; and in our 

 survey of the heavens now drawing to a close, we shall 

 have done what we can to show how it may be made to 

 do so. 



THE FISHERIES EXHIBITION. 



By John Ernest Ady. 



TO understand the structure of the sponges in general 

 our best plan would be to trace the life-history of 

 some typical form from its embryonic origin to the adult 

 condition. But this is a matter which is involved in the 

 greatest difficulty, because there are, so to speak, many 

 typical forms, which, so far as they have been studied, 

 present wide differences from one another. Then there are 

 other well-known examples, such as the common fresh- 

 water sponge, which present phases of development now 

 considered to be aberrant. 



We are thus driven to the necessity of taking an hypo- 

 thetical case, and, in following its growth, we shall point 

 out the deviations which lead to the several types which 

 occur in nature. 



Fig. 11. — Diagrams of the Deteiopjiext of a Sponge : a, ovum 

 and a', spermatozoa ; h, fertilisation of ovum by spermatozoa ; c, 

 resultant morula, witli blastocoele c' ; d, one pole of the embryo 

 ciliated, the other end spiculigerous ; e, the ciliated pole invaginated 

 into the non-ciliated part ; /, aperture of embryo closed, showing 

 epiblast with sijiciiles, and hypobl.ist with cilia surrounding a blas- 

 tocoele; 17, the stage / fixes itself to a foreign object, and develops 

 an aperture or osculum at the other end ; the epiblast fuses into a 

 syncj'tium, and the hypoblast becomes converted into the endo- 

 dorm ; h, the young sponge now develops inlialent pores, which 

 place its ventriculus (v) in communication with the exterior. The 

 arrows indicate the course of the currents of water which flow in 

 at these pores, and escape from the eshalent aperture or osculum. 

 (Original.) 



It is now an all but universally acknowledged fact that, 

 as Martin Barry first pointed out,* cells do not arise de 

 novo, as was long ago supposed, but derive their existence 

 from pre-existing cells. To pass backwards in this inquiry, 



* Phil. Trans., Royal Society, London, 183S-9. 



and to ask for an explanation of the ultimate origin of the 

 living cell, and the nature of life, would be equivalent to 

 an entry into the domain of ni'-taphysical speculation, 

 which we, as physical beings, must altogether decline. 



We therefore start with certain cells of the adult sponge 

 which become modified at particular periods into rounded 

 masses bounded externally by a firm portion, and con- 

 taining what is termed a nucleus and a nucleolus within. 

 The whole cell thus formed separates from the parent 

 organism, and is called an ovum or female element 

 (Fig. 11, rt). In yet other cells of the adult sponge, the 

 protoplasm breaks up into small particles, which upon 

 maturity emerge from the so-called mother cell of evolu- 

 tion, and acquire peculiar whip-like tails (Fig. 11, a) 

 and clear hyaline heads ; these are the spermatozoa, 

 or male elements. Ere long, the spermatozoa pene- 

 trate the ovum (Fig. 11, h), and become absorbed in 

 some mysterious manner. After this fertiUsatimi the 

 ovum undergoes peculiar changes ; its nucleus (germinal 

 vesicle), and nucleolus (germinal spot) become obscured, 

 and its protoplasm breaks up by repeated division into 

 a mass of nucleated cells, termed a mulberry mass, or, 

 technically, a morula (Fig. 11, c), within which there is a 

 cavity or blastocode. One half of this mass of cells produce 

 cilia (Fig. 11, (/) or lash-like processes, and by means of 

 these the little embryo swims about. After a time the 

 ciliated cells become tucked (or invaginated) into the other 

 part, somewhat after the fashion of the drawing-in of the 

 tip of a glove-finger, and the stage now arrived at (Fig. 11, e) 

 has been termed by Haeckel the gastrula stage. Now, it 

 is a curious fact, according to that distinguished observer, 

 that certain sponges [O'astrophysema, Ilaliphysema) get no 

 farther than this stage, which (the gastrula) is also a strong 

 argument in favour of the classification of the sponges 

 amongst the ccolenterate animals, instead of with the 

 lowlier unicellular creatures which are classed together 

 as Protozoa. Indeed, so very important is this phase of 

 existence amongst animals, that it forms the line of 

 distinction between the Protozoa and all the higher groups, 

 which have, on that account been called, collectively, 

 Metazoa. 



It was a theory of Haeckel's that the mulberry mass of 

 cells (Fig. 11, c), formed a central ca-saty by an internal 

 splitting or delamination, and that an external aperture 

 afterwards appeared to complete the gastrula stage ; but 

 the researches of Mctschnikoff,* corroborated by F. E. 

 Schulze, have led himf to abandon that aspect of the case, 

 and to agree generally with what has been sketched above, 

 which is an abstract of Metschnikoft''s views. 



The gastrula now consists of two layers of cells 

 (Fig. 11, ('), an oxtornal layer (epiblast) and an internal 

 layer (hypoblast) ; w ithin these there is the primitive 

 cavity or blastocwle. The mouth now closes (Fig. 11, /), 

 and the outer layer of cells gradually becomes fused to 

 form a membrane in which the individual cells cannot 

 be readily distinguished ; and in this .^i/ncijtium the skeletal 

 elements begin to be formed. The embryo has now passed 

 into the condition of a young sponge ; it fixes itself by one 

 end to some foreign object, and develops an aperture or 

 oscubim at the other extremity (Fig. 11, g). Next, the 

 cells of the body-wall separate at irregular intervals to 

 form pores (Fig. 11, h) which can be extemporised at any 

 part of the body, or which may as readily be effaced ; and 

 thus a simple sponge is the result, which resembles the 

 adult organism in every essential particular. 



* " Zur Entwicke ungsgeschichte der Kalkschwamme, Zeita. fiir 

 wias. Zoologie," Bd. xxiv. 



t "Die Gastrula und die Eifurchung der Thiere," p. 158. 



