116 BULLETIN OF THE 



phiura by Metschnikoff (op. cit. PL III. fig. 6),* and that the two struc- 

 tures v., supposed by him to be water tubes, correspond in position to 

 the chisters of cells on each side of the invagination. These clusters in 

 Ophiopholis were quite dense, and the vesicles, if they existed here, 

 would be difficult to see. Metschnikoff says that in Amphiura these 

 bodies are also difficult to see through the " Cutiszellen " (mesoderm 

 ceUs), and that later in normal development one is lost. He was able to 

 observe that one of these bodies in Amphiura develops into the water 

 tubes of the adult. It is not wholly certain that similar bodies do not 

 exist in Ophiopholis, PI. I. fig. 14, a cl., where clusters of amoeboid cells 

 make observation on live material somewhat difficult at these points. 



The bilateral arrangement of tlie budding cells in the cavity of the 

 blastosphere and the shape of the larva give to it a marked bilateral sym- 

 metry even at this early stage. The pole of invagination may be called 

 an anterior pole, while the cells on each side indicate the sides of the 

 larva. One hemisphere of the gastrula is flattened ; the opposite is 

 more rounded. The former may be called the ventral, the latter the 

 dorsal surface. 



At seven o'clock on the day following the spawning the invagination, 

 which forms the archenteron, has extended about half way down the 

 cavity of the blastosphere, PI. I. fig. 15. Almost the whole of the 

 second twenty-four hours is occupied by the changes which accompany 

 the infolding of the archenteron. f 



The pole of the infolding slowly sinks into the cavity, carrying with it 

 at this point the shell of cells, or that part of the blastoderm which is to 

 form the wall of a digestive canal. The larva has become very much 

 flattened on the ventral side, so that when seen from the pole of invagina- 

 tion the lateral diameter is twice that at right angles to it in the same 

 plane. As we have arbitrarily called the longest diameter, when seen 

 from the pole of the blastopore, the lateral, a name which seems appro- 

 priate, not only on account of the bilateral symmetry which the larva at 

 this early age h^ 3, but also from the fact that from its extremities form 

 the two calcareous rods and fleshy arms, known as the lateral arms, we 

 may speak of the other diameter as the dorso- ventral. The dorso- ventral 

 diameter connects the dorsal and ventral side of the larva, which are 



* Fig. 6 is a little older. The mode of origin of these vesicles was not observed 

 by MetschnikoflF. Their position relatively to the mouth of the larva is somewhat 

 exceptional. 



t The time occupied to form the gastrula of Ophiophragma is about the saiae as 

 in Ophiopholis. Cf. Nachtrieb, op. cit. 



