384 



TUNICATA. 



the rudiment of the central nervous system, which here does not take 

 the form of a tube but of a solid ectodermal growth, while a ventral 

 cell-mass (rh) is assumed to be the rudiment of the chorda and a 

 posterior mass the mesoderm (ms). According to Uljanin, the arch- 

 enteron is used up in the formation of the chorda and the mesoderm : 

 the rudiment of the adult intestine on the contrary owes its origin 

 to an independent ectodermal invagination which occurs later. 



In the next stage (Fig. 178 C), the embryo appears to be folded 

 several times within the egg. A dilated anterior region principally 



Fig. 179. — Two so-called larval' stages of Doliolum Mulleri (after Uljanin). A, 

 younger stage ; Ji, older stage, ch, cliorda ; cl, atrial cavity ; eb, ectodermal vesicle ; 

 h, heart ; m, egg-shell ; ms', anterior mesoderm-rudiment ; ms", posterior mesoderm- 

 rudiment ; n, nervous system : /i. pharynx ; x, mesodermal rudiment of the stolon ; 

 //, mesoderm-masses. 



occupied by the large ganglionic rudiment (n) ran be distinguished 

 from a caudal region bent back upon itself, in which the chorda (ch) 

 is scon to be already developed. Two lateral mesoderm-bands (ms) 

 run along the whole length of these two regions of the body. Similar 

 stages were also observed by Fol (No. 21). 



In the stages which follow (Fig. 179 A), the embryo straightens 

 itself out within the egg-shell and is now able to raise itself from 

 the bottom of the sea on which the egg rests and to swim about 



