1913.] 



Protostigmata in Ascidians. 



445 



1. Grotvth. — This takes place chiefly at the ends of the stigmata, and 

 results in elongation. 



2. Perforation. — This is the appearance of an opening in a fused area of 

 the pharyngeal and atrial walls. It may occur early or late. 



3. Subdivision. — This is the division of a stigma by the growth across it of 

 a bridge. It may occur early or late in the growth of a stigma ; it may 

 occur at a varying point on the stigma ; it may occur before or after 

 perforation. 



4. Orientation or Direction of Growth. — This factor determines the shapes 

 and the positions of the stigmata with reference to each other. It depends 

 upon equality or inequality of growth of the cells on either side of a stigma. 

 Great variation is shown in this factor. 



By variations in these four factors there results the infinite diversity in the 

 stigmata of Ascidians. A comparatively slight variation may make a great 

 difference in the final picture, and a similar end result may be attained by 

 following quite different paths. One should avoid giving too much signifi- 

 cance to certain differences shown. It is certainly astonishing to see, in a 

 young colony of Botryllus, the first blastozoid provided with four rows of 

 minute definitive stigmata that have arisen by independent perforation, and 

 the oozoid beside it with six large protostigmata that never become converted 

 into definitive stigmata. 



The Origin of the Stigmata in the Various Groups. 



In fig. 2 the methods of development of the protostigmata in the various 

 groups are schematically represented. They all are seen to be derived from 

 a common starting point, a single stigma or rudiment not yet perforated. 

 The stigmata represented in solid black are those that have not yet perforated. 

 Many of them are hypothetical. Usually a dark mass can be seen at the 

 point where a stigma will shortly perforate. 



Perhaps the simplest method is the fourth of the figure, that occurring in 

 the Botryllidas and some Styelidie. The protostigmata appear in order 

 beginning in front. Each stigma is formed by independent perforation at 

 a point behind the middle of the previously formed one. Julin (1904«) has 

 maintained that in Dendrodoa (Styelopsis) grossidaria the protostigmata arise 

 in a manner analogous to that of Ciona described above. Both Damas (1904) 

 and Fechner (1907) describe their origin by independent perforation and 

 give excellent figures. In an allied species {Dendrodoa earned) I have found 

 no indication of intercalation or of new protostigmata being formed from 

 pre-existing ones, the conditions being the same as figured by Damas and 

 Fechner. 



