THE DEVELOPMENT OP ASTERINA GIBBOSA. 227 



and in fig. 24 we see that the ccelom has grown back in the 

 form of two tongues, Ipc, rpc, lying one at each side of the 

 gut. Fig. 35 shows us a more ventral section passing through 

 the blastopore of the same individual, and we see that in it 

 these ccelomic lobes are absent ; they are therefore still con- 

 fined to the dorsal side of the embryo. 



It has been mentioned above that the larva, immediately on 

 escaping from the egg-membrane, has the form of Stage B, and 

 it will be observed that its anterior end has the appearance of 

 being obliquely truncated, and that the anterior surface so 

 constituted is surrounded by a thickened rim, which is covered 

 with specially long cilia, and to which I give the name of larval 

 organ. The changes of form involved in acquiring this shape 

 are considerable, and are undergone whilst the larva is still 

 enclosed in the egg-membrane, though superficially the ovoid 

 shape is maintained, the larval organ and the neighbouring 

 ectoderm being to a large extent developed as invagina- 

 tions into the interior of the larva, exactly as the 

 Taenia head is developed on the wall of the cyst. 



The histology of the embryo is illustrated in Plate XIX, figs. 



124 and 125. The first is a portion of section of a larva of 

 Stage A, the same specimen as that from which figs. 20 and 21 

 are taken. Both ectoderm and endoderm are seen to consist of 

 long narrow cylindrical cells, and there is no mesenchyme. 

 Recent researches have gone to show that this is exceptional. 

 Field (5) has proved for Asterias, and it has been long known 

 in the case of Echinids, that mesenchyme is formed by the wall 

 of the blastula before any invagination has taken place. Fig. 



125 is taken from a slightly older gastrula. It shows the forma- 

 tion of the mesenchymatous cells by the division of the endoderm 

 cells. I found no indication that mesenchyme continued to be 

 formed when Stage B is reached. The anterior wall of the 

 ccelom is the spot where its formation lasts longest, as in 

 Antedon (18). The ccelomic epithelium consists of small cubical 

 cells (see PI. XVI, fig. 95). 



We must now return to Stage B, up to which we have traced 

 the development. A stomodseum is now developed just behind 



