1892.] Development of the Stigmata in Ascidians. 



507 



stigmata in Pyrosoma clearly tends to show that these are simple 

 structures, and that they have not arisen by the modification of so 

 many transverse rows of stigmata, as his theory demands. 



The question is thus seen to be still unsolved, and if in this commu- 

 nication I venture to offer some observations upon the matter, it is 

 in the belief that they tend considerably to elucidate the problem. 



The view that the pelagic Caducichordate Tunicata have been pro- 

 foundly modified in structure through their mode of life, and that they 

 are to be derived phylogenetically from the so-called Compound Asci- 

 dians, is at present held, with varying reservations, by almost every 

 recent investigator of the Tunicata, except Seeliger. It is held by 

 Grobben and Uljanin for Pyrosoma, Salpa, and Doliolum, by Herdman 

 for Pyrosoma, by Lahille for Pyrosoma and Doliolum, and by Salensky 

 for Pyrosoma and Salpa. The evidence for this view has always seemed 

 to me to be very slender and unimportant ; and I believe it is this 

 widely-spread conception which is answerable, among other things, 

 for the absence of any satisfactory comparison between the stigmata 

 of the fixed Ascidians and of their pelagic allies. 



I have accordingly approached the question from the reverse point of 

 view, believing that, by a study of the development of the stigmata in 

 the fixed Ascidians, recapitulative stages would be met with which 

 would furnish the desired answer. A grant awarded me by the 

 Government Grant Committee last year enabled me, during the 

 summer, to collect suitable material at the Plymouth station, and my 

 observations have been completed in Professor Milnes Marshall's 

 laboratories at the Owens College. 



The development of the stigmata in the larva and oozooid of 

 Ascidians has hitherto been very little investigated. The earliest 

 complete account is that of Krohn,* an abstract of whose observa- 

 tions upon Phallusia mammillata is given by Balfour (' Comp. Emb.,' 

 vol. 2, p. 20). "Krohn's interpretations were subsequently criticised 

 by E. van Beneden and Julin in their valuable paperf on the " Post- 

 embryonic Development of Phallusia (Ascidiella ?) scabro'ides." These 

 investigators showed that in the latter species two stigmata at first 

 appear, one behind the other, on each side of the pharynx, and that 

 subsequently new stigmata arise between and behind the two first, 

 until a longitudinal row of six stigmata is formed on each side. 

 These stigmata enlarge transversely to the long axis of the body, and 

 subsequently subdivide, in the order of their formation, so as to con- 

 stitute a corresponding number of transverse rows of smaller stigmata. 

 Van Beneden and Julin thus drew a distinction between primary 

 stigmata and secondary (definitive) stigmata, and called attention to 

 the irregular order of formation of the primary stigmata as a point 



* " Ueber die Entwicklung d. Ascidien," ' Muller's Arcbiv,' 1852. 

 f « Arch, de Biologie,' vol. 5, 1884, p. 611. ' 

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