MOLLUSCA. 255 



half of the ovum then divide more rapidly than the remainder ; 

 there is in this way formed an elongated sphere, half of which is 

 composed of small cells and half of larger cells. In the interior 

 is a small segmentation cavity. From its eventual fate the 

 hemisphere of the smaller cells may be called the anterior pole, 

 and that of the larger cells the posterior. An involution of the 

 cells at the apex of the posterior pole (though not of the whole 

 hemisphere of larger cells) now takes place, and gives rise to the 

 archenteron. At the same time an equatorial double ring of 

 large cells appears on the surface between the two poles, which 

 becomes ciliated and forms the velum. At the apex of the an- 

 terior pole a tuft of cilia, or at first a single flagellum, is estab- 

 lished (fig. 116 III. and IV.). 



In the succeeding developmental period the blastopore, which 

 has so far had the form of a circular pore at the posterior ex- 

 tremity of the body, undergoes a series of very remarkable 

 changes. In conjunction with a gradual elongation of the larva 

 it travels to the ventral side, and is prolonged forwards to the 

 velum as a groove. The middle part of the groove is next con- 

 verted into a tube, which opens externally in front, and post- 

 eriorly communicates with the archenteron. The walls of this 

 tube subsequently fuse together, obliterating the lumen, and 

 necessarily causing at the same time the closure of the blasto- 

 pore. The tube itself becomes thereby converted into a plate 

 of cells on the ventral surface between the epiblast and the 

 hypoblast 1 . 



While the above changes have been taking place the meso- 

 blast has become established. It is derived from the lateral and 

 ventral cells of the hypoblast. 



After the establishment of the germinal layers the further 

 evolution of the larva makes rapid progress. A transverse 

 groove is formed immediately behind the velum, which is 

 especially deep on the ventral surface ; and the stomodaeum is 

 formed as an invagination of the anterior wall of the deeper 

 section of the groove. Behind the stomodaeum the remainder 

 of the ventral surface grows out as a flattened foot. 



1 There is a striking similarity between the changes of the blastopore in Chiton 

 and the formation of the neurenteric canal of Chordata ; especially if Kowalevsky is 

 correct in stating that the pedal nerves are developed from the ventral plate. 



