418 THE EYE OF THE TUNIC ATA. 



nasal cavity. The cord so formed gives rise to the whole of the duct 

 proper and to the lower branch of the collecting tube. The upper branch 

 of the collecting tube is formed as an outgrowth from this cord. A lumen 

 begins to be formed on the twelfth day of incubation, and first appeai-s at 

 the nasal end. It arises by the formation of a space between the cells of 

 the cord, and not by an absorption of the central cells. 



In Mammalia Kcilliker states that he has been unable to observe any- 

 thing similar to that described by Born in the Sauropsida and Amphibia, 

 and holds to the old view, originally put forward by Coste, that the duct is 

 formed by the closure of a groove leading from the eye to the nose between 

 the outer nasal process and the superior maxillary process. The upper 

 extremity of the duct dilates to form a sack, from which two branches 

 pass off to open on the lacrymal papillse. In view of Bom's discoveries 

 Kolliker's statements must be received with some caution. 



The Eye of the Tunicata. 



The unpaired eye of the larva of simple Ascidians is situated some- 

 what to the right side of the posterior part of the dorsal wall of the 

 anterior cephalic vesicle (fig. 296, 0). It consists of a refractive portion, 

 turned towards the cavity of the vesicle of the brain, and a retinal por- 

 tion forming part of the wall of the brain. The refractive parts consist 

 of a convex-concave meniscus in front, and a spherical lens behind, 

 adjoining the concave side of the meniscus. The posterior part of 

 this lens is imbedded in a layer of pigment. The retina is formed 

 of columnar cells, with their inner ends imbedded in the pigment 



Fig. 296. Labva of Ascidia mentula. (From Gegenbaur; after Kupffer.) 



Only the anterior part of the tail is represented. 



iV'. anterior swelling of neural tube; N. anterior swelling of spinal portion of 



neural tube; n. hinder part of neural tube; ch. notochord; K. branchial region of 



alimentary tract; d. oesophageal and gastric region of alimentary tract; 0. eye; 



a. otolith; o. mouth; s. papilla for attachment. 



which encloses the posterior part of the lens. The retinal part of 

 the eye arises in the first instance as a protninence of the wall of 

 the cerebral vesicle: its cells become very columnar and pigmented 

 at their inner extremities (fig. 8 V, a). The lens is developed at a 

 later period, after the larva has become hatched, but the mode of its 

 formation has not been made out. 



