PROTOSTKIMATA OF MOLGULA MANHATTENSIS. 145 



mitted the siinultaiieons ovio'in of the two first-foniied o-iH- 

 clefts. 



2 . T e t r a s t i g" m a t i c Stage. — This stage has been observed 

 by Krohn (1852) for Phallusia mammillata, by Ed. van 

 Beneden and Julin (1884) for Ascidiella scabroides, and 

 by me (1892) for Ciona intestinalis, and again (1893) for 

 Molg-ula manhattensis (see below). 



The inauguration of this stage or the transition from the 

 preceding to this had not been observed before my account 

 of the process in Ciona. 



The ventral ends of the two primordial stigmata of each 

 side bend towards one another, and their walls even come into 

 contact ; each of them, then, forms by constriction a small 

 diverticulum, that from the first stigma being directed back- 

 wards, and that from the second stigma forwards. The two 

 diverticula from the primordial stigmata thus come to lie in 

 the primary interstigmatic space and are finally cut off from 

 the parent stigmata. In this quite unexpected way is the 

 tetrastigmatic stage of Ciona intestinalis introduced. 



The second stigma of the distigmatic stage becomes, after 

 the abstriction of the two intercalary stigmata, the fourth of 

 the series of protostigmata in the present stage. 



;> and 4. Peuta- and lie xa st igmatic Stages. — '^l''hese 

 can l)e considered together, l)ecause in Ciona intestinalis 

 the fifth and sixth ])rotostigmata arise successively bv inde- 

 pendent jierforatioii. A pentastigmatic stage was described 

 by Krohn for Phallusia mammillata and a hexastigmatic 

 stage by van Beneden and Julin for Ascidiella scabroides. 



The description of the hexastigmatic stage of the last-named 

 species given by van Beneden and Julin was the only one A\'ith 

 which I could compare the corresponding stage observed by 

 me in C i o n a. In making the comparison I was influenced by 

 Krohn's statements regarding Phallusia mammillata, and 

 consequently assumed too great a uniformity in the method 

 of formation of the protostigmata of simple Ascidians. 



Van Beneden and Julin found that the protostigmata of 

 Ascidiella scabroides were of unequal and irregular sizes 



VOL. 44, PART 1. NEW SERIES. K 



