xvi ECHINODEKMATA 545 



Antedon, unlike Holothuroidea, lives in the open and can usually 

 be obtained, when it is found at all, in enormous numbers by means of 

 the dredge. At proper seasons of the year, ranging from May in the 

 Mediterranean to July in the Clyde, a few ripe males and females are 

 certain to be included in the haul. These, when placed in clean 

 sea-water in the aquarium, will emit sperm and eggs early in the 

 morning, about 7 A.M. 



Both Bury and Seeliger found the best preservative to be a 

 concentrated solution of corrosive sublimate in sea-water, to which a 

 small proportion (Bury used ^, Seeliger ^) f glacial acetic acid has 

 been added. For the study of the development of the calcareous 

 plates the embryos must be preserved in spirit alone. 



The fertilized eggs adhere to the under side of the pinnules of 

 the parent by means of a glutinous egg-membrane, and from these 

 eggs free-swimming larvae emerge on the fifth day. 



The fertilized egg divides into two and then into four equal seg- 

 ments ; in the 8 -cell stage, however, the four blastomeres at the 

 animal are smaller than the four at the vegetative pole. The blasto- 

 coele appears as a narrow central separation of the blastomeres in 

 the 4-cell stage, which in the 8- and 16-cell stages is wider at the 

 vegetative end. In the 32-cell stage it becomes closed at the animal 

 end by the displacement of the blastomeres ; in this stage the vegeta- 

 tive pole is surrounded by eight specially large blastomeres. In 

 the 48-cell stage these eight blastomeres approach one another and 

 close the opening at the vegetative pole. These eight blastomeres, 

 however, subsequently divide, so that when 128 cells are formed and 

 the complete blastula stage is attained no difference in the size of 

 its component cells can be detected. 



The blastula stage is attained in about eight hours. Then it 

 becomes converted into a gastrula by the appearance of a pit on its 

 surface and the invagination of a portion of its wall to form the 

 lining of the archenteron. This pit or blastopore is not circular in 

 outline but slit-like, and the long axis of this slit is at right angles to 

 the plane of symmetry of the gastrula, which is no longer quite 

 spherical but slightly elongated in one direction. The slit becomes 

 crescentic, with the horns curved backward, but the archenteron is 

 curved forward. 



As soon as the archenteron is partly formed numerous mesenchyme 

 cells are given off from its free apex and wander into the blastocoele. 

 These cells arise in two ways; either a cell in the wall of the 

 archenteron buds off a mesenchyme cell, or else it escapes bodily from 

 between its neighbours in the archenteric wall. As the invagination 

 proceeds the blastopore becomes reduced in size till it forms a small 

 pore, and finally, when development has gone on for thirty-six 

 hours, it is closed, although a groove marks for some time the spot 

 where it existed. 



The archenteron is flattened in the plane of the blastopore, i.e. 

 in the transverse plane. In the second night, i.e. in about forty 



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