34 



exterior tbrougb the atrial apertures. Opinions differ as to 

 whether only one or a few pairs of true gill clefts are 

 represented in the young Ascidian, and the actual details 

 of their formation and sub-division differ greatly in differ- 

 ent forms. To what precise extent the walls of the atrial 

 or peribranchial cavities are formed of ectoderm or endo- 

 derm, is also still doubtful. 



The embryo is hatched, about two or three days after 

 fertilisation, as a larva or Ascidian tadpole (fig. 9) which 

 leads a free-swimming existence for a short time during 

 which it develops its nervous system and cerebral sense 

 organs, and the powerful mesoblastic muscle bands lying 

 at the sides of the notochord (now a cylindrical rod of 

 gelatinous nature surrounded by the remains of the 

 original cells) in the tail, and forming the locomotory 

 apparatus. Figure 9 shows this stage, the highest in its 

 chordate organisation, when the larva swims actively 

 through the sea by vibrating its long tail provided with 

 dorsal and ventral fins. 



In addition to the structures already mentioned, the 

 mesoderm has formed the beginning of the muscular body- 

 wall and the connective tissue around the organs, and has 

 given rise to the blood, the endostyle has developed as a 

 thick-walled groove along the ventral edge of the pharynx, 

 which now becomes the branchial sac, and the pericardial 

 sac and its invagination the heart have formed in the 

 mesoblast between the endostyle and stomach. The 

 unpaired optic organ in the cerebral vesicle, when fully 

 formed, has a retina, pigment layer, lens and cornea ; while 

 the ventral median sense-organ is a large spherical, partially 

 pigmented otolith, supported by delicate hair-like processes 

 on the summit of a hollow "crista acustica " (fig. 9). 

 Both the otolith and the retina and lens of the eye are 

 formed oiigmally by the differentiation of a group of cells 



