102 ART. 4. — N. YATSU : 



Lingula the archenteron is in many cases obliterated, while in 

 other cases it persists as a wide cavity. This difference is of 

 special interest on account of the fact that the developmental 

 mode of Linyula is intermediate between that of Cistella and 

 of Lacazella. The difference cannot be considered as one of 

 kind but only of degree owing to the rapidity of cell increase at 

 various parts. 



The closure of the blastopore occurs both in Lingula and 

 Cistella. 



In Lingula the mesoblast is formed as a pair of cell masses 

 proliferated from the lateral walls of the archenteron and the 

 body cavity by a splitting of the same cell masses. Lingula, 

 therefore, is a deuleroccelier (Ziegler), whose body cavity is 

 formed after the mode of schizocœl 1 (Huxley, Roule). The 

 formation of the mesoblastic cell masses of Lingula closely re- 

 sembles that of Lacazella ; in the latter type, however, it was not 

 observed that the mesoblast was proliferated from a thickening of 

 invaginated entoblast walls. Kowalevsky ('83) states merely : 

 " bientôt des cellules provenant probablement des premières rem- 

 plissent cette cavité," (p. 69). From this statement some authors 

 conclude that the entoblast and mesoblast in Lacazella are formed 

 by the process of delamination. It is more probable that the 

 process is a polar in-growth (Einwücherung). At any rate the 

 result of the mesoblast formation, expressed by Kowalevsky in 

 the sentence : " l'intérieur se partage alors en trois lobes " has 

 not a little similarity to that of Lingula. 



The formation of the body cavity after the enterocœlic type 



1. The terra " schizocoel " has been employed in two quite different meanings. Huxley 

 ('75) means "a perivisceral cavity formed by a splitting of the mesoblast" as in Annelids, 

 while the Hertwigs ('81) use the term as a synonym of " pseudocœl " (p. 13). (Of 

 Ziegler '98 p. 27 and p. 34). 



