MUSEUM OF COMPARATIVE ZOOLOGY. 333 



represented in Fig. 27 is shown, and, in order to exhibit the relations 

 of the various parts more clearly, it is figured in the position which 

 the latter would occupy if it were rotated 90° upon its axis. The 

 side, instead of the edge, of the inner partition is therefore seen, and 

 one of the sinus tubes (i), with its enclosed ovary (y), is above, and 

 the other below it. The edges of the partition soon begin to thicken 

 and spread out, so that a section would present nearly the shape of a 

 letter H, with a cavity in the cross-bar. The four flaring edges 

 now divide up into four rows of bud-like prominences, and two of 

 these extend into the space between the constrictions which mark out 

 the body cavity of each zooid, and lengthen tmtil they meet upon 

 the median line and surround the egg, which is now found in this 

 position, as shown in Fig. 29. In this figure the prominences as well 

 as the egg seem to be in contact with the outer tunic ; but as it was 

 necessary to use pressure in order to obtain a side view, this appear- 

 ance is to be explained as caused in this way, for at a stage only a 

 little later, but sufficiently advanced to be studied without pressure 

 (Fig. 30), a very distinct body cavity is seen separating them from 

 the body wall. In this figure the portion which surrounds the egg 

 (s) has now separated from the portions upon each side of, or more 

 strictly before and behind it, and forms the egg capsule. The two 

 remaining portions (5 and 6) have also separated from the partition, 

 and form oval masses, which are free within the body cavity of the 

 zooid. One of these (5) is destined to form the ganglion, and the 

 other (6) the branchial sac and digestive organs. They are very sim- 

 ilar in shape and size, and each contains a cavLy entirely surrounded 

 by a thick wall. These cavities are, without driiibt, at first divertic- 

 ula from the cavity of the inner tube or partition, and are invisible 

 in Fig. 29, on account of the pressure to which the specimen there 

 figured was subjected; but the early changes in these parts take place 

 so rapidly that no specimen was found at a stage which admitted of 

 the determination of this point, although hundreds were examined 

 with this end in view. The portion which is to form the digestive 

 organs increases in size much more rapidly than the ganglionic por- 

 tion, and extends forward under the egg and ganglion, to the anterior 

 end of the body, and thus gives rise to the branchial sac. The sub- 

 sequent development of these parts docs not differ essentially from 

 that of the same parts in the solitary embryo. 



