64 SUMMARY OF CURRENT RESEARCHES RELATING TO 



portion of tlie primitive mesenteron. It is certain that the anus does not 

 represent the posterior extremity of the second portion of the mesenteron. 

 It is necessary to distinguish clearly between the descending portion of the 

 tract developing out of the primary rudiment, and the ascending portion 

 arising as secondary caecum from the floor of the stomach. In every way 

 the adult ClavelUna is seen to be absolutely like the visceral portion of the 

 larval trunk, extended lengthwise after the atrophy of the tail. 



IV, The reproductive system. — In this chapter the development of the 

 reproductive organs is described in Perophora, Clavtllina, and Phallusia. 

 The genital rudiment is of mesodermic origin, and appears in the con- 

 cavity of the intestinal curve. The same rudiment forms testis, ovary, 

 and ducts. There is no distinct ovarian rudiment, nor are there two 

 cellular cords, but always only one. The female organ of a Perophora or 

 ClavelUna is simply an embryonic vesicle consideral)ly elongated, in which 

 the epithelial wall forms in certain regions ovarian follicles. A secondary 

 diverticulum arising from the floor of the primitive vesicle is the first 

 rudiment of a testis. The various stages are described in detail. 



As to oogenesis, Kowalevsky's early conclusions are confirmed. The 

 egg-envelope ai:)pears after the follicular cells have disposed themselves 

 in two layers. It is therefore of epithelial origin and separates the pri- 

 mitive epithelium into two portions, — the test-layer, and the secondary 

 follicular epithelium. A subsequent subdivision of tliis external layer, 

 about the time of egg-laying, results in the formation of two layers — one 

 adjacent to the egg-membrane and homologous to the papillary or spumose 

 layer of other Ascidians, the other opposed to the " anhyste " membrane 

 described by Fol in Ciona as the membranous follicular layer. The primi- 

 tive follicular epithelium formed by a simple layer of flat cells is derived 

 from follicular cells interposed between the primordial ovules in the ger- 

 minal epithelium. This epithelium gives rise to two " membranes anhystes," 

 and to three cellular layers, viz. from the outside inwards — (1) the mem- 

 brane anhyste of the follicle, (2) the membranous follicular layer, (3) the 

 papillary or spumose layer, not so in ClavelUna, (4) the "membrane 

 anhyste " of the egg-envelope (Fol's cliorion), (5 ) the testa layer. Of 

 these 1 and 2 remain till the egg is laid ; 3, 4, and 5 are expelled along 

 with the egg. The latter is naked and gives rise to no membrane. The 

 envelopes which surround it after, as before laying, are of follicular 

 origin. 



Y. The fifth chapter discusses the longitudinal muscles. In their struc- 

 ture they rather resemble the primitive bundles of vertebrate muscle than 

 the cellular fibres ; there is, however, no cross striping. 



The remaining hundred pages are occupied with the discussion of 

 general morphological questions raised by the above investigation. The 

 development of ClavelUna is very intimately compared with that of Amphi- 

 0XU8, the question of the vertebration of the larvfe is fully discussed, the 

 morphology of the heart, branchial slits, and various cavities is critically 

 resumed, and the interpretation of Tunicata as degenerate fish is unfavour- 

 ably criticized. 



As to the systematic position of Tunicata, the authors conclude their 

 valuable memoir as follows : — (1) The Tunicata, Cephalochordata, and 

 Vertebrata form a single group— Chordata. (2) The Tunicata have, like 

 the other two divisions, arisen from segmented enterocoelous organisms, 

 like the Archiannelid worms. Animals like Protodrili, but with dorsal 

 chord and anterior respiratory diverticula from the gut, formed the com- 

 mon starting-point for the Chordata. In these Protochordata the posterior 

 portion of the trunk is adapted more especially for locomotion, while the 



