A. KROHN ON THE DEVELOPMENT OP THE ASCIDIANS. 321 



With respect to the first stages of development, I have not 

 obtained all the information I desired, on account of the many 

 difficulties in the way of a satisfactory examination. Never- 

 theless I will not hesitate to communicate, in a condensed form, 

 the observations which belong here, imperfect as they may be. 

 It will be observed that as regards many points, they harmonize 

 with the views of Van Beneden. 



Soon after the three hollow processes, the rudiments of the 

 future vessels of the test, have appeared, we distinguish in the 

 body, besides those parts which belonged to the larva, viz. the 

 two pigment spots, now closely approximated, and the coiled tail, 

 a cavity which represents the future respiratory or gill-sac. 

 Immediately behind this appears the first indication of the ali- 

 mentary canal, which appears in the form of a canal, everywhere 

 of even diameter and bent round into the form of a loop. The 

 whole posterior division of the body is filled by the coil of the 

 tail. 



Subsequently one may observe on the dorsal surface, on the 

 second layer of the body, which lies under the mantle and is 

 clearly distinguished, three apertures, over which the mantle, 

 still imperforate, passes. The one lies exactly in the middle 

 at the anterior extremity of the body, the two others are alto- 

 gether lateral and diametrically opposite to one another, in the 

 middle portion of the body. The anterior, somewhat larger, 

 corresponds with the future ingestive or respiratory aperture ; 

 the two posterior are intended to take the place of the future 

 cloacal aperture, which arises, at a much later period, by their 

 coalescence. At this time the nervous ganglion also has become 

 developed, and may be readily distinguished as an elongated 

 structure in the middle of the dorsal surface and close above 

 the two pigment spots. Near it we see the first indications of 

 the future interwoven muscular cords of the body ; the abdo- 

 minal furrow is already indicated; the nutritive canal has 

 become further developed ; it lies now, for the most part, above 

 the respiratory sac and describes a curve, commencing at the 

 bottom of the latter and passing round to the immediate neigh- 

 bourhood of the left excretory aperture. Three divisions may 



during the development of Amouroucium proUferumy and which he has fully 

 described. 



SCIEN. MEM.— iV^a<. Hist. Vol. I. Part IV. 21 



