36 ZOOLOGY SECT. 



neural gland early appears on the ventral wall of a ciliated 

 diverticulum (cil. gr.) of the anterior end of the archenteron 

 (future pharynx), which subsequently unites with an outgrowth 

 from the medullary canal. 



The embryonic alimentary canal consists of two regions, a wide 

 region situated altogether in front of the notochord, and a nar- 

 rower portion situated behind in .the region of the notochord. The 

 wider anterior part gives rise to the pharynx, the posterior part 

 to the oesophagus, stomach, and intestine. The mouth-opening is 

 formed shortly before the escape of the embryo from the egg ; an 

 ectodermal invagination is formed at the anterior end, and an 

 endodermal diverticulum from the archenteron grows out to meet 

 it ; the two coalesce, and the oral passage is thus formed. 



The rudiments of the heart and pericardial cavity first appear 

 as a hollow outgrowth from the archenteron : this subsequently 

 becomes constricted off and involuted to form a double- walled sac, 

 the inner layer of which forms the wall of the heart, while the 

 outer gives rise to the wall of the pericardium. 



The first beginnings of the atrial cavity appear as a pair of 

 invaginations of the ectoderm which grow inwards and form a 

 pair of pouches, each opening on the exterior by an aperture. 

 There is a difference of opinion as to some points in the 

 history of these atrial pouches, and it remains uncertain to what 

 extent the ectoderm and endoderm respectively share in the 

 formation of the atrial cavity. Eventually spaces, into the forma- 

 tion of which the two ectodermal diverticula at least largely 

 enter, grow round the pharynx and give rise to the atrial cavity ; 

 and perforations, the stigmata, primarily two in number, place the 

 cavity of the pharynx in communication with the surrounding 

 space. The two openings of the atrial pouches subsequently 

 coalesce to form one the permanent atrial aperture. 



It will be useful now, at the cost of a little repetition, to sum- 

 marise the various characteristics of the larval Ascidian at the 

 stage when it escapes from the egg and becomes free-swimming 

 (Fig. 735). In general shape it bears some resemblance to a 

 minute tadpole, consisting of an oval trunk and a long laterally- 

 compressed tail. The tail is fringed with a caudal fin, which is 

 merely a delicate outgrowth of the thin test covering the whole 

 of the surface ; running through the delicate fringe are a series of 

 striae, presenting somewhat the appearance of the fin-rays of a 

 Fish's fin. In the axis of the tail is the notochord (noto.}, which 

 at this stage consists of a cylindrical cord of gelatinous substance 

 enclosed in a layer of cells. Parallel with this runs, on the dorsal 

 side, the narrow caudal portion of the nerve-cord, and at the sides 

 are bands of muscular-fibres. In the trunk the nerve-cord is 

 dilated to form the ganglion of the trunk, and, further forwards, 

 expands into the sense-vesicle (sens, ves.) with the otocyst (or stato- 



