3 20 ANIMAL BIOLOGY. [Part II. 



The single yolk-sphere will give rise to hypoblast and mesoblast. 

 The large yolk-segment next undergoes cleavage, until about ten 

 blastomeres are formed from it, two of which are larger than 

 the others. These two slip into the cavity of the blastosphere, 

 while the others flatten out and undergo involution so as to form 

 a gastrula, with an elongated blastopore. A considerable space 

 separates the archenteric wall of the gastrula from the wall of 

 the blastosphere. From the two larger cells which first slipped 

 inwards a number of small cells are budded off, which form a 

 layer applied to the archenteric wall, and subsequently develop 

 into a network which fills up the space between this wall and 

 the wall of the blastosphere. 



The embryo now rapidly develops into a peculiar larval form 

 originally regarded as a parasite of the mussel known as the 

 glochidium. In the winter the external gill of the female mussel 

 will be found to be distended with immense numbers of these 

 glochidia, together with yellow granular food material. Fig. 94, 

 7-11, shows their form and structure. Each has a pair of valves 

 perforated with minute apertures, united along a straight hinge 

 line, and connected by a single adductor muscle. When 

 examined alive they may be seen to open and shut as they lie 

 within the vitelline membrane. The edge of the shell opposite 

 the hinge is produced into an incurved beak, set with sharp 

 spines. Long coiled threads- the byssus may be seen in some 

 cases hanging down from the glochidium. Within the valves 

 is seen a mass of cells forming the embryonic mantle-lobes, and 

 careful examination will disclose three pairs of peculiar tactile 

 organs, consisting of columnar cells, bearing numerous fine 

 bristles. 



The glochidia remain for, a long while in the gill of the parent. 

 But if some fish (e.g. perch or sticklebacks) be placed in the 

 tank in which such brood-mothers are living, the embryos are 

 ejected in great numbers. In some cases (in all 1) they are 

 ejected through the pore marked^?, in Fig. 90, A. andB., which 

 is placed at the dorsal end of a canal, the ventral end of which 

 opens into the exhalent siphon of the supra-branchial chamber. 

 The glochidia are then set free from the egg-membrane, and 



