DEVELOPMENT OF GANGLION AND NEURAL GLAND. 



23 



or from one of its anterior limbs. The heart arises as an in- 

 vagination of the dorsal wall of the pericardium (Fig. 17, pc). 

 The tunic is laid down as a cuticular excretion of the ectoderm, 

 and the larva is hatched. After a few hours of free life, during 

 which it swims by the movement of its tail, it attaches itself by 

 the three anterior ectodermal papillae and the tail shrinks up to 

 a stump and atrophies (Fig. 18). The cerebral vesicle largely 

 breaks down. The dorsal part of it which communicates with 

 the pharynx by the new neuropore persists and gives rise to the 

 neural gland. The dorsal wall of this thickens and forms the 



-_j .___ ur . -,^* " 



e-y pp ft, 



FIG. 17. Left side view of a Clarelina embryo (after Seeliger from Korschelt and Haider* 

 uu eye ; ch notochord ; e atrial aperture ; ed rectum ; ep epicardial outgrowth of pharynx 

 es endostyle ; / folding of the body surface in anticipation of the rotation that take 

 place after fixation ; fg duct of neural gland ; h adhering papillae ; i mouth ; ks gill slits ; 

 m stomach ; mz muscle cells of the tail ; oe oesophagus ; ot auditory organ ; p atrial cavity ; 

 pc pericardium ; s larval tail ; sb cerebral vesicle. 



ganglion of the adult. If this account is correct it follows that 

 the neural gland is a part of the original cerebral vesicle. It 

 has been compared to the hypophysis (pituitary body) of the 

 Vertebrata ; but it differs from this in the fact that it is, in its 

 origin, actually a part of the embryonic brain, which the pit- 

 uitary body never is. The trunk ganglion becomes solid and 

 persists as the visceral nerve. At the same time the fixed larva 

 undergoes a peculiar change which is illustrated in the annexed 

 diagram (Fig. 18). The mouth which was close to the point of 

 attachment becomes displaced to the opposite end, and the 

 original hind end becomes placed close to the point of attach- 

 ment. In a few monascidians (Culeolus, etc.) this change does 



