416 ZOOLOGY SECT. 



ap.cil.}. Encircling the body of the larva behind this is a thick- 

 ened ridge, the prototroch (prof.), the cells of which develop strong 

 cilia. Just behind the prototroch the cells of the ectoderm 

 become pushed inwards, in the middle of what will eventually 

 become the ventral surface, so as to line a sort of depression or 

 pouch : this is the stomodceum (st.) or rudiment of the mouth and 

 oesophagus. The anus (an.) does not appear until later ; the position 

 which it will subsequently occupy is indicated at this stage by a 

 pigrnented area (pig. ar.) marking the point at which the blasto- 

 pore becomes closed. The first and second somatoblasts divide 

 to form a mass of small cells which extend on the ventral surface 

 behind the prototroch and mouth, constituting what is termed 

 the ventral plate; of this plate the more superficial cells are 

 descendants of the first somatoblast one of the twelve original 

 micromeres ; and those situated more deeply are derived from 

 the second somatoblast or mesomere. A superficial thickening 

 along the middle of the ventral plate is the rudiment of the 

 ventral nerve-cord (neur. pi.) ; the deeper cells divide and extend 

 to form a pair of mesoderm bands or muscle plates, from which 

 the muscles of the body-wall are developed ; the muscular layers 

 of the wall of the alimentary canal are derived from certain of the 

 same set of cells which migrate inwards from the lower end. 



A pair of micromeres separated from the rest at an early stage 

 are destined to form the larval excretory organs, the head kidneys : 

 at first situated at the upper end, they sink below the surface and 

 migrate downwards till they come to lie below the prototroch ; 

 each then elongates, and a number of vacuoles which have become 

 formed in the interior coalesce in such a way as to form a long 

 narrow canal. The embryo has now reached the completed Tro- 

 chosphere stage. 



The endoderm cells become arranged so as to bound a canal- 

 like space, the beginning of the lumen of the middle part of the 

 alimentary canal (oesophagus and intestine) (int.), the cells subse- 

 quently giving rise to the enteric epithelium. This canal becomes 

 continuous in front with the stomodseum, and behind with a 

 second smaller ectodermal invagination, the proctodceum, which 

 arises in the position of the former pigment area. The part of 

 the larva behind the prototroch now elongates, and two pairs of 

 imaginations, the setigerous sacs (set. sacs), appear at its sides: in 

 the interior of these, to which a third pair is soon added, are 

 developed setae which grow out to a great relative length as the 

 larval or provisional seta:. Constrictions soon appear marking off 

 the first three segments, and at the same time the mesoderm bands 

 undergo a corresponding division into three pairs of mesoderm 

 segments. The mesoderm segments of each pair grow inwards 

 towards one another and surround the alimentary canal ; in the 

 interior of each appears a cavity which is the beginning of a 



