194 



HARD WICKE'S SCIENCE- G OSSIF. 



free ; each consists of a little highly-refractile conical 

 head, with a long tail, which vibrating rapidly, 

 propels the spermatozoon, as the structure is called, 

 head foremost through the water. 



The spermatozoon is the male element, the ozntm the 

 female element of the sponge. Both occur in the 

 mesoderm. It is a singular thing that the male 

 elements have not yet been discovered in Sycandra 

 raphamis, but they have been seen in various other 

 sponges ; in some species occurring in the same 

 individuals as contain the ova, in others in difterent 

 individuals, so that some sponges are hermaphrodite, 

 and others bisexual. 



If the ova and spermatozoa be kept from each 

 other nothing remarkable will happen, each will lead 

 its own life, and and die a natural death. But they 

 do not as a rule remain apart ; the spermatozoa 

 when set free on the disruption of the sperm-balls, 

 are carried away in the out-flowing water from the 

 radial tubes of one individual, to enter along with 

 the incurrent water the radial tubes of another. 

 Should this latter contain ova, the spermatozoa on 

 approaching an ovum swim towards it like so many 

 tadpoles, and striking it head foremost, with their 

 tails streaming outwards, remain for a while radiately 

 disposed about its circumference. Finally they enter 

 the substance of the ovum, are absorbed and disap- 

 pear. The ovum is then said to be impregnated. 

 Unfortunately this process, though it has been 

 witnessed in many other sponges, has not been 

 observed in Syeandra raphamis. As a consequence of 

 the fusion of the spermatozoa with an ovum, the latter 

 undergoes a remarkable series of changes, which end in 

 producing a fresh sponge like the parent. First of 

 all the nucleus disappears, and after awhile two 

 fresh nuclei appear in its place ; a constriction then 

 occurs round the exterior of the ovum, as a shallow 

 furrow, which deepens, extending inwards between 

 the two nuclei till it actually divides the ovum into 

 two parts, each of which has the value of a true cell. 

 The direction in which this division takes place is 

 constantly from the top of the ovum (that side 

 facing the endodermal layer), to the bottom, or, as 

 we may briefly formulate it, perpendicular. The cells 

 resulting from the division are flattened below and 

 also against each other, but rounded off towards the 

 top, thus diminishing in size from the base upwards. 

 They soon undergo the same kind of change as the 

 parent ovum, the nucleus of each disappears, and is 

 replaced by two fresh nuclei, each is then furrowed 

 by a constriction, and eventually divided into two 

 cells. The plane of division is still perpendicular, but 

 at right angles to the previous one. The four cells 

 thus resulting resemble the parent cells in being 

 broad and flat below, and narrower towards the top ; 

 they are likewise flattened against each other, but 

 rounded off along their inner perpendicular edges, so 

 as to leave a small cavity or canal in the axis of the 

 group. This canal is the beginning of what we shall 



know hereafter as the cleavage cavity. Division 

 again takes place, and still in a perpendicular 

 direction ; each of the four cells is divided into two 

 from top to bottom, and thus eight cells arise, which 

 form a ring (fig. 94 a), surrounding the cleavage 

 cavity (ib.), and narrowing from the base upwards. 

 After these three perpendicular divisions, which have 

 given us first two, then four, and finally eight, cells, 

 a fourth one occurs, which is horizontal, and so 

 at right angles to all the preceding ones. As the tops 

 of the cells, previous to this division, were, as stated 

 before, smaller than the bottoms, so it follows that 

 the eight upper cells above the median plane of 

 division are smaller than the eight lower cells 

 beneath it. The embryo consists now of sixteen cells 

 in two rows (fig. 94 c), an upper or apical row, and 

 a lower or basal row, of eight cells each, surrounding 

 the cleavage cavity, which opens by a wide lumen 

 below, and a much narrower aperture above. The 

 embryo exchanges now its plano-convex, or cake-like, 

 for a cushion-like, or biconvex, form. Two more 

 divisions, also in a horizontal direction, now succeed, 

 severing each ring into two, and thus producing a 

 four-ringed form ; one ring is apical, one basal, and 

 the two between may be called equatorial. The 

 cells of the equatorial rings are again divided, and 

 this time vertically, in a meridional direction, we 

 might say ; in this way each equatorial row comes 

 to contain sixteen cells. The embryo now is a 

 cushion-like sac, the wall being composed of a single 

 layer of similar cells, forty-eight in number, sixteen 

 in each equatorial ring, and eight in each polar 

 ring. The number of cells is further increased 

 by division, the sac becomes more spherical in 

 form, and the apical end of the cleavage cavity 

 closes up. The next stage in the development is a 

 differentiation of the cells, the eight which surrounded 

 the basal opening of the cleavage cavity, and which 

 are larger than the rest, acquire a dark appearance, 

 owing to the development of a great number of dark 

 granules within them. The remaining tells ai-e 

 much clearer, and multiplying in number, are con- 

 verted into elongated small prism-like cells, radially 

 arranged. The basal end of the cleavage cavity now 

 closes up, and the dark granular cells increase in 

 number. The next stage is either abnormal, incon- 

 stant, or in any case subsequently reversed ; in it the 

 layer of granular cells becomes flattened, depressed, 

 and then pushed into the cleavage cavity, which is 

 thereby diminished, or even almost obliterated, the 

 granular cells applying themselves to the inner face 

 of the prismatic layer (fig. 94). The form so 

 produced is similar to the gastrula of other animals, 

 but it is not permanent, the granular cells with- 

 drawing themselves, and subsequently resuming their 

 former position. 



We may, therefore, return to the larva, where we 

 left it in the normal course of development. The 

 embryo is now more or less egg-shaped ; the smaller 



