No. I.] GERM-LAYERS IM CLEPS/NE. 123 



" Dr. Kleinenberg has followed a single egg through the whole 

 course of its development, and concludes that the nuclei of 

 Nephelis never become the nuclei of new cells." ^ 



Assuming, then, that the ten terminal cells in Nephelis are the 

 homologues of the teloblasts in Clepsine, and that, accordingly, 

 the primitive mesoblastic cords discovered by Biitschli arise 

 neither from the micromeres nor from the three entoblastic 

 macromeres, a, b, c, but from a pair of cells {x and xy) 

 derived from the posterior macromere, xx, the way is clear for 

 comparing the entoderm cells. The chief differences are : (i) 

 the absence of " free nuclei" in Nephelis, and (2) the contra- 

 distinction in position of the three macromeres, which lie 

 within the mesenteron in Clepsine, and external io it in the body 

 cavity in Nephelis. The foundation of both differences is 

 undoubtedly the relative abundance of food-yolk. The egg of 

 Clepsine is much larger than that of Nephelis, and is completely 

 filled with yolk-spherules, which are to serve as food during 

 several weeks of larval life. The egg of Nephelis, on the other 

 hand, has very little food-yolk, the larva depending upon the 

 fluid albuminous substances contained in the egg-case. The 

 abundance of yolk in the egg of Clepsine makes it necessary for 

 the nuclei to seek a peripheral position, as in the case of so 

 many arthropod eggs ; and thus the yolk is left within the 

 enteric epithelium. In the egg of Nephelis, the entoderm cells 

 all escape from the macromeres at an early date, and henceforth 

 they multiply by subdivision, and probably grow at the expense 



' Balfour has here given Kleinenberg credit for what neither he nor any one else has 

 ever yet accomplished. The method has not yet been discovered by which the egg of 

 Nephelis can be kept alive for more than a few hours, after removal from the egg- 

 case. Continuous observation on a single egg through all its stages of development 

 is, therefore, a feat entirely beyond our present means. Kleinenberg's opinion as 

 to the fate of certain nuclei must rest on evidence of a much less conclusive nature 

 than supposed by Balfour. It is to be hoped that Kleinenberg will yet publish the 

 results of his study on Nephelis, for we certainly stand in need of more light on this 

 subject. 



Bergh's brief remark on the origin of the mesenteron may be interpreted in favor 

 of the existence of free nuclei. " Letzteres [Mitteldarmepithel] bildet sich namlich 

 aus dem primaren Entoderm durch fortdauernde Kernverf7tehrung ; erst nach dem 

 Ausschllipfen aus dem Kokon bildet es sich als eigentliches Epithel aus, indem dot 

 Protoplasma sich urn die Kerne herum in Zellen sondert." (No. 13, p. 294.) 



(13.) Bergh, R. S. Ueber die Metamorphose von Nephelis. Zeitschr. f. wiss. 

 Zool. XLI. H. 2. p. 284. 1884. 



