ASCIDIACEA — DEVELOPMENT OF THE FREE-SWIMMING LARVA. 3(>T 



from the eye, i.e., as a rule, on the left side, although Lakille 

 (No. 37) considers that this condition varies in the different forms. 

 Lahille (No. 37), Sheldon (No. 52), Willey, (No. 54), and 

 Hjort (No. 59), on the contrary, have been led by their researches, 

 to confirm in almost all points the older observations of Ko\vai,k\ 

 sky as to the rise of the ciliated pit, Le., t<> regard it, in its origin, 

 as much more closely connected with the central nervous system. 

 After the neuropore has completely closed, the most anterior section 

 of the cephalic vesicle lengthens and fuses with an ectodermal 

 depression, the stomodaeum (rudiment of the larval mouth, branchial 

 aperture). At this point perforation takes place (Figs. 167 A, 168 

 .1,/), so that now the cephalic cavity, by means of this short tube 

 which represents the rudiment of the ciliated pit, communicates with 

 the most anterior ectodermal section of the alimentary canal. Only 

 in later stages when, after the fixation of the larva, the larval 

 nervous system degenerates, is the connection between the ciliated 

 pit and the nervous system lost. The pit then forms a blind in- 

 testinal diverticulum contiguous with the definitive ganglion (Fig. 

 173.7/).* 



According to these statements, the ciliated pit opens into the ectodermal 

 or stomodaeal portion of the branchial sac, and we thus have a condition 

 agreeing with that of the hypophysis in the Vertebrata. 



The chorda. The chorda arises through the transformation of a 

 plate-like rudiment (Fig. 159, ch), which originally formed the roof 

 of the archenteron, into a cell-strand which, in cross-section, is 

 round (c/. Figs. 160 and 161, ch). We have seen above (p. 348) 

 that we must suppose this to have been brought about by the forma- 

 tion of a groove (as in Amphioxus). In cross-section, the chorda-strand 

 is originally composed of several cells. Both in lateral (Fig. 162 A) 

 and in dorsal aspect it appears composed of two rows of cells, the 

 ends of which dove-tail with one another. This dove-tailing of 

 the cells denotes the commencement of a change of position which in 

 most cases leads to the chorda-cells appearing- arranged one behind 

 the other in a single row like a roll of coins (Fig. 163 A). In those 



* [ Willey (No. XXXVI.) has recently reinvestigated this point both in 

 (',10)1(1 and Clavelina. He is convinced that van Beneden and Julih were 

 quite mistaken in their interpretation of the origin of the hypophysial tube 

 in Clavelina. The whole is derived essentially from the neural tube, and 

 thus the lumen of the hypophysis is at Hist in direct communication with 

 the lumen of the central nervous system, the opening of this structure into 

 the pharvnx being, according to Willky. a reopening of the neuropore. — 

 Ed.] 



