II LARVAL DEVELOPMENT 59 



from the much narrower posterior part of the enteron, which 

 grows out to become the oesophagus, stomach, and intestine. The 

 notochord does not extend forward into the pharyngeal region, 

 but is confined to tlie posterior or caudal part of the embryo. It 

 now shows lenticular pieces of a gelatinous intercellular substance 

 secreted by the cells and lying between them (Fig. 25, I). The 

 mouth forms as a stomodaeum, or ectodermal invagination, 

 antero-dorsally in the region where the neuropore has closed, and 

 about the same time two lateral ectodermal involutions form 

 (Fig. 26, A, at), which become the atrial or peribranchial 

 pouches, at first distinct, afterwards united in the mid-dorsal line 

 to form the adult cloaca and atrial aperture. Ingrowths from 

 the atrial pouches and outgrowths from the wall of the pharynx 

 coalesce to form the proto-stigmata (primary gill-slits) by which 

 the cavity of the branchial sac is first placed in communication 

 with the exterior through the atrial apertures. Opinions differ 

 as to whether only one or a few pairs of true gill-clefts are repre- 

 sented in the young Ascidian ; and the actual details of their 

 formation and subdivision, to form the stigmata of the adult, 

 differ considerably in different forms. In Clavelina the stigmata 

 are formed as independent perforations of the pharyngeal wall ; 

 in Ascidia two pairs of protostigmata increase to six pairs, which 

 are subdivided into stigmata ; JBotryllus and other forms are 

 intermediate in some respects. No doubt the subdivision of proto- 

 stigmata is primitive, but has been lost from the ontogeny in 

 some cases. To what precise extent the walls of the atrial or 

 peribranchial cavities are formed of ectoderm, or of endoderm, is 

 still doubtful. 



The embryo is hatched about two or three days after fertilisa- 

 tion, as a larva or Ascidian tadpole (Fig. 26, A) 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 which form the 

 locomotory apparatus. Fig. 26, A, shows this stage, the highest 

 in its chordate organisation, when the larva swims actively 

 through the sea by vibrating its long tail with the dorsal and 

 ventral fins. 



In addition to the structures already mentioned, the mesoderm 



