444 



Mr. A. G. Huntsman. 



[Mar. 3, 



In E is shown the condition in some Tethyidae (Boltenia) and some 

 Styelidae (Styela). There are four secondary, followed by an indefinite 

 number of primary. 



F represents the condition in some Styelidae (Dendrodoa {Styelopsis)) and 

 in the Botryllidae. There are an indefinite number of primary and no 

 secondary protostigmata. Julin has given an entirely different account of 

 the origin of the protostigmata in Dendrodoa {Styelopsis) from that given by 

 Damas. My investigation of this genus is in accord with the account given 

 by Damas. It seems possible that Julin's material has been wrongly 

 identified as belonging to this genus. 



Damas (with Garstang and Seeliger) considers the single row of stigmata 

 in the Pyrosomatidae and Doliolidae as being a series of protostigmata. Julin 

 (with Lahille) considers the row as homologous with one of the transverse 

 rows of stigmata in an Ascidian. I favour the latter view. Damas' objection 

 that the longitudinal bars of Pyrosoma would, in that case, not be homologous 

 with those of Ascidians is valid, but they are readily homologised with the 

 internal transverse vessels of Holozoa and of many simple Ascidians, as done 

 by Lahille (1890). 



The series of fig. 1 shows gradation in two respects. There is a continuous 

 increase in the number of primary protostigmata, and, excepting A, there is 

 a continuous decrease in the ability of the protostigmata to subdivide, this 

 decrease being greater posteriorly than anteriorly. The variation shown in 

 the families Tethyidae and Styelidae is noteworthy. The former family 

 contains some forms with a limited number (three) and others with an 

 indefinite number of protostigmata. Up to the present, a relatively small 

 proportion of the existing genera have been investigated. Further studies 

 will doubtless show even greater variations. 



The Factors of Stigmatogenesis. 



In interpreting the various stages that may be examined, it is important to 

 keep in view the factors involved in the process. The Ascidian pharynx 

 offers a splendid field for a study of the processes of development, since great 

 variations are shown in the different groups, and the processes are practically 

 confined to two dimensions, i.e. one plane. 



As the basis of the process we have, of course, the properties of the cells 

 and their interaction with each other and with the surrounding water. 

 These determine the ways in which the cells divide and arrange themselves 

 to give .the gross characters that are seen. 



Certain gross factors (depending upon the above), which vary in the 

 different groups, may be mentioned. 



