PEOTOS'I'IGMATA OF MOI>GULA MANHATTENSIS. 151 



The fact that the growth of stigma A is in advance of that 

 of B is a detail of some interest in comparison with the 

 behaviour of the homologous stigmata in C ion a intestinalis^ 

 where B tended to take the priority of A. 



Thus stigmata A and B produce by abstriction two neAv 

 stigmata, C and D. These latter arise, as the figures show, 

 not in the form of minute diverticula of A and B, as is the 

 case in Ciona, but as large reduplications equal in longitu- 

 dinal diameter to the parent clefts, though less in their trans- 

 verse (dorso-ventral) diameter. Moreover, whereas in Ciona 

 the stigma D was the third in the definitive series of proto- 

 stigmata, here it becomes the fourth of the series, owing to 

 the fact that its abstriction from B takes place in the opposite 

 direction to that observed for Ciona. 



3. Pent astigmatic Stage. — I regret that among the 

 drawings which I still possess relating to M. manhattensis 

 I can find none illustrating this stage, so that I must refer to 

 the composite diagram reproduced on PI. 9, fig. 10, which 

 was drawn at the time that the observations were made in 

 1893.1 



After tlie establishment of the tetrastigmatic stage by the 

 abstriction of C and D from A and B, m fifth protostigma, E, 

 arises by independent perforation. In M. manhattensis, 

 by observing many specimens, noting the curvature of the 

 already formed stigmata (cf. fig. 11), and seeing a new stigma 

 at its first appearance as a minute perforated disc, it is possible 

 to demonstrate beyond reasonable doubt that the fifth proto- 

 stigma is formed independently of the rest. This being the 

 case, it is the more singular to find that the fifth protostigma, 

 E, subsequently behaves in exactly the same manner as A 

 and B did, namely, it doubles round upon itself at its ventral 

 end, and the recurved arm is the primordium of the sixth 

 and last protostigma, F. 



4. Hexastigmatic Stage (PI. 9, figs. 11 and 12).— The 

 appearance of the protostigmata shortly after the abstriction of 

 the sixth from the fifth (F from E) is sho'^^^l in fig. 1 1 . Here it 



^ Ci. ' Aiiiphioxus and tlie Aucestrv of the Vertebrates/ lS9i, pp. 232-3. 



