204 THE ASCIDIANS. 



With the formation of the tail the enteric cavity be- 

 comes confined as a closed sac to the anterior portion of 

 the embryo. It is bounded dorsally by the nerve-tube, 

 which is somewhat dilated in this region, and in front, at 

 the sides and below, it is in close contiguity with the 

 ectoderm. 



Formation of the Adhesive Papilla. 



At a much later stage than that represented in Fig. 101, 

 the ectoderm bounding the convex anterior extremity of 

 the body becomes raised up into three prominences, whose 

 relations to one another are those of the corners of a tri- 

 angle. They are due to the ectodermic cells at the respec- 

 tive points assuming a high columnar shape. They become 

 eventually raised very much above the adjoining surface of 

 the ectoderm, and become the adhesive papilla or fixing 

 glands of the larva. The cells composing them acquire the 

 power of secreting a viscid substance, by which the larva 

 can fix itself to any favourable surface (Fig. 102). 



Cerebral Vesicle and its Sense-organs. 



We have spoken above of the dilated anterior portion of 

 the nerve-tube. This is the part of the central nervous 

 system which undergoes the most striking subsequent 

 changes. By a gradual widening of its cavity, accom- 

 panied by a local thinning out of its wall, this portion 

 of the neural tube lying in front of the notochord becomes 

 transformed into a spacious sub-spherical vesicle, known 

 as the cerebral vesicle (Fig. 102). 



While the anterior portion of the neural tube is enlarg- 

 ing to form the cerebral vesicle, granules of black pigment 

 are deposited by certain cells in the dorsal wall of the 

 vesicle. The granules are at first scattered about in the 



