318 A. KROHN ON THE DEVELOPMENT OF THE ASCIDIANS. 



withers away by degrees in such a manner that the lobes 

 become smaller and fewer, until they are but minute remnants. 

 In the end these also disappear. When the coil begins to 

 diminish, it occupies, as before, the whole posterior division of 

 the body of the developing Ascidian ; but subsequently, when 

 it has become visibly smaller, it lies more on the left side near 

 the oesophagus (PL XII. B. fig. 3). 



From what has been said then, it results, that Van Beneden 

 has not rightly comprehended the appearances during the extri- 

 cation of the tail from its investment, since he considers this act, 

 which for the rest his figures depict very well, to depend upon 

 an absorption of the tail. Had Van Beneden recognized all the 

 phases of the retrogressive metamorphosis of the tail, he could 

 not long have remained in doubt as to the true nature of the 

 undefined organ (/. c. pi. 3. figs. 11 & 12 e) met with in the 

 posterior part of the body of Ascidia ampulloides during the first 

 periods of development. He would have at once seen it to be 

 the coiled-up and already lobulated tail. 



Very soon after the tail has been withdrawn into the body, 

 and the latter has thence even to some extent increased in size, 

 the three processes of attachment of the larva disappear, while 

 in the meanwhile the mantle of the developing animal is fastened 

 to the ground by its whole lower surface. From the mass of 

 the body, and indeed from the midst of the abdominal surface, 

 three processes of another nature now grow forth, and pene- 

 trating deeper and deeper into the mantle, attain its surface. 

 These hollow processes are the first indications of that dichoto- 

 mously branched vascular system, with distinct walls, which pe- 

 netrates the mantle of all Phallusice^. These processes are soon 

 seen to become longer and to divide dichotomously into the first 

 branches, whose extremities appear clavate; the dichotomous 

 branching goes on further and further, and the terminal branches 



* According to Kollikev's more exact investigations (/. c), this vascular 

 system consists everywhere of double vessels, which running close together 

 ramify correspondingly, and accompany one another to the very finest ter- 

 minal ramifications, when a mutual anastomosis takes place. According to 

 my observations, this anastomosis occurs in such a manner that the terminal 

 twigs of the two vessels bend round into one another like a loop. All these 

 observations are also, as we shall see, perfectly confirmed by the study of deve- 

 lopment. 



