326 NIKOLAS KLEINENBERG. 



becoming shallower as it approaches the ventral surface 

 (PL X, fig. 22). At the same time that this fossa is being 

 excavated, the simple layer of ectoderm covering the cephalic 

 germinal streak folds itself round the edge of the projection 

 and is reflected into the buccal fossa, which till now was 

 lined with large endoderm cells (PL IX, fig. 10 ; PL X, 

 figs. 22, 23, 24, eo). The inbending begins at the dorsal 

 surface, and extending from here embraces little by little the 

 sides, and finally the ventral portion of the fossa. Thus, the 

 ingestive canal, which anteriorly represents the mouth, but 

 posteriorly is converted into the oesophagus, becomes covered 

 by a plaster of ectoderm cells instead of its original 

 endodermic covering, which is thrust towards the bottom of 

 the digestive cavity. The newly formed epithelium of the 

 mouth and of the oesophagus consists of a single layer of 

 slightly granular, cylindrical cells, elongate in the interior, 

 but becoming shorter as they approach the edge of the fold, 

 where they are continuous with the external covering of the 

 body. They soon put out vibratile cilia, very similar in form 

 and movement to those already described on the ventral 

 surface. A similar covering of cilia extends now also over a 

 circle of the external ectoderm surrounding the mouth. 



The vibratile cilia in the embryo of Liimhricus are thus 

 confined to the tract of ectoderm cells Avhich is situated on 

 the ventral surface between the germinal streaks, and 

 extends from the mesoblast cells at the aboral pole to the 

 cephalic extremity, where it unites with the vibratile ring 

 just described. This mode of distribution of the ciliated 

 cells, which remains unaltered for nearly the whole of 

 embryonic life, calls to mind that, not of the larvse properly 

 so called, but of the young stages of many, and the adult of 

 not a few, Chaetopods. 



It is obvious that, as the germinal streaks lengthen, their 

 respective positions, as well as the general shape of the 

 embryo, must alter, and the necessary changes take place, as 

 is always the case in the mechanism of the animal body, 

 according to the principle of least resistance. In fact, 

 instead of producing directly the lengthening of the embryo 

 (to which, perhaps, the endoderm and ectoderm, which at 

 this period only follow passively the movements of the 

 germinal streaks, would offer too much resistance) ; the 

 streaks seek to meet the increasing need of space by leaving 

 their lateral symmetrical positions and placing themselves on 

 the convexity of the ventral surface, about a radius of 

 curvature which constantly becomes smaller ; at the same 

 time their points of origin, that is to say, the two large cells. 



