ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 63 



terior portion is directly transformed into muscular cells. Thougli the 

 segmentation of the tail is not apparent, it is none the less real. (13) The 

 alimentary canal of the larva is straight and median. It is continued by a 

 neurenteric canal into the medullary tube ; but both this and the caudal 

 portion of the intestine disappear. The remainder includes three distinct 

 portions : (a) the precordal dilatation, (b) a shrunk region, of gutter-like 

 form, open above, and roofed by the notochordal plate, (c) a long rudimen- 

 tary portion, without cavity, lying beneath the notoch.ord as a double row 

 of endodermic cells. 



II. The heart, the pericardium, and the epicardiac tube. — The open 

 tubular heart has a delicate muscular wall, connected along one line, by a 

 suture or raphe, with the pericardiac epithelium round about. The cardiac 

 cavity arises as an invagination of a portion of the pericardiac wall. They 

 thus really form one sac. The heart is but the visceral layer of the peri- 

 cardium. If it were not for the absence of a vascular endothelium the 

 ascidian heart would be directly comparable to that of a vertebrate embryo. 

 The epicardium associated with the heart forms a blind tube, forked in 

 front, and opening by two distinct orifices, right and left, into the branchial 

 cavity between the entrance to the oesophagus and the posterior extremity 

 of the hypobranchial groove. The cavities are lined by an epithelium con- 

 tinuous with that of the branchial sac, at the level of the orifices. The 

 whole arrangement is intimately described. The epicardium separates 

 the two principal currents of blood — the poster o-anterior, ventral, hypo- 

 branchial, or sub-epicardial, from the antero-posterior, aortic, or supra- 

 epicardiac current. The epicardium and pericardium, which are thus 

 physiologically and anatomically associated, arise genetically from the 

 same embryonic formation — the procardium. From the development of 

 this procardium, which is discussed in detail, it is evident that heart, peri- 

 cardium, and epicardium all develope at the expense of the branchial 

 endoderm. It is further noteworthy that the procardium arises from a 

 double rudiment, and that the right portion is smaller than the left. 



In the bud the internal vesicle results from the separation of the two 

 cellular layers adjacent to the stolon partition. The elongated vesicle is 

 divided transversely into terminal and basilar portions, of which the former 

 gives rise to branchial sac and epicardial tube, and to secondary diverticula, 

 the peribranchial cavities and digestive tube. The basilar portion forms 

 the pericardial sac, of which the invaginated roof becomes the wall of the 

 heart. The subsequent changes are described at length, and the authors 

 show that the mode of development, and anatomical relations of the cardiac 

 organs, are different in the forms arising from a urodelous larva, from 

 what they are in those arising from buds. The heart of the adult is then 

 described. 



III. The third chapter contains an account of the development of the 

 alimentary tract. The three regions of the larval tract have been noted 

 above. That of the adult arises wholly from the two first portions of th.e 

 larval mesenteron. The authors' observations show that the branchial sac, 

 the oesophagus, and the stomach are differentiated portions of the one pri- 

 mitive larval rudiment. These three portions have remained median and 

 symmetrical. The stomach ending in a cul-de-sac is prolonged backwards 

 m a short stout cellular cord, the remnant of the caudal portion of the 

 primitive mesenteron. The intestine arises in the form of a secondary 

 diverticulum from the floor of the stomach. It is a new and superadded 

 structure. Its mode of development recalls that of glands. The intestinal 

 caecum arises to the right at the level of the anterior extremity of the 

 notochord. It originates rather from the precordal, than from the subcordal 



