874 SUMMARY OF CURRENT RESEARCHES RELATING TO 



is a circumpolar zone. The rays or fibrils which are inserted into the 

 surface of the egg along the polar and circumpolar grooves form two 

 lines, one polar and one circumpolar ; the fibrils produce the cones, 

 the apices of which correspond to the centre of attracting spheres. 

 They form with the achromatic half-spindles a whole. When 

 daughter-nuclei are formed and gain the surface of the blastomeres 

 the principal cones — as the half-spindles may be called — and the anti- 

 podal systems disappear. The cause of cell-division seems to reside 

 in the protoplasm, and the separation of the secondary chromatic disks 

 from one another is an effect of the same kind as the appearance of 

 the superficial antipodal systems. In a table the authors give a view 

 of the filiation between the different cells at different stages of 

 Begmentation. 



Relation of the Nervous System of the Adult Ascidian to that of 

 the Tailed Larvse.* — E. van Beneden and C. Julin contribute a 

 detailed memoir upon this subject. For their investigations they 

 made use of the larva of Clavelina rissoana, the earlier phase of which 

 they had already studied. 



The central nervous system is composed of (1) a cerebral vessel 

 bearing the organs of sense, (2) a visceral portion reaching to the 

 commencement of the tail, and (3) a caudal portion ; all are traversed 

 by a central canal dilated into a vesicle in the cerebral and further 

 back in the visceral portion ; these divisions of the nervous system are 

 not peculiar to the larvae of Clavelina, but have been shown by others to 

 exist in Salpa and Pyrosoma ; in the adult the caudal portion disappears 

 entirely, while only a part of the cerebral and visceral portions 

 remain ; the parts that remain are those which in the fully mature 

 larva have retained their embryonic character and are formed by a 

 simple epithelium, i. e. the cerebral cul de sac and the visceral canal ; 

 the parts that are already differentiated in the larva, that is the sense- 

 organs and the delicate epithelial wall of the vesicle together with the 

 ganglionic mass adjacent to the floor of the visceral canal disappear. 



Although nothing is known respecting the development of Appen- 

 dicularia, there seems no doubt that the cerebral organ of that animal 

 corresponds to the interoscular ganglion of the Ascidian and that the 

 nervous cord traversing the tail corresponds to the caudal portion in the 

 urodele larvae. M. Fol, however, considers the central nervous cord 

 of Appendicularia as a simple nerve. 



The paper terminates with some remarks upon the formation of 

 the branchial apparatus : it appears that the peribranchial cavities (in 

 PeropJiora) are developed, as Kovalevsky showed, from the arehen- 

 teron ; in Clavelina these same cavities originate from the epiblast 

 and become connected with similar outgrowths of hypoblast into 

 which they open ; this temporary condition is permanently retained 

 in Appendicularia ; in the adult Ascidian it is therefore clear that the 

 peribranchial cavities are the homologues of the endodermal part of 

 the branchial slits in Appendicularia. The development of the peri- 

 branchial cavities of Ascidians is precisely similar to that of the gill- 



* Bull. Acad. K. Belg., viii. (1884) pp. 13-72 (4 pis.). 



