230 NIKOLAS KLEINENBERG. 



of L. trapezoides, because the great curvature of the ante- 

 rior end of the body, would easily conceal the existence of a 

 very narrow fissure. Notwithstanding this, I am convinced 

 that the splitting of the mesoderm appears first in the 

 cephalic germinal streak ; that, namely, the cephalic segment 

 is the first formed, although the first segment of the trunk 

 is formed nearly at the same time. 



The splanchnic layer of the cephalic ring, which at first 

 covers only the upper side of the buccal fossa and oesophagus 

 with a thick layer of mesoderm, extends gradually its lateral 

 ^j- parts towards the central surface, and embraces the ingestive 

 aperture completely. Then certain cells of its deeper layer 

 begin to migrate into the inflected ectoderm which 

 clothes the cavity of the head intestine, making their 

 way between the bases of the epithelial cells and slightly 

 raising them (PI. II, figs. 19 h,e>d, 23, 24). This process 

 begins also from the dorsal side, and ends in the formation 

 of strong and thick walls for the head intestine, which by 

 their origin belong to the splanchnic layer of the mesoderm, 

 from which they become distinctly divided. The epithelium 

 becomes reduced to a thin almost cuticular membrane, which 

 in the adult state lines the mouth and oesophagus. 



Thus, the walls of the ingestive end of the alimentary 

 canal, at three successive periods of embryonic life, have a 

 structure different both in form and in the origin of the 

 material ; at first they are formed of endoderm, this then 

 becomes pushed away and replaced by an inflection of 

 the external covering of the body, and, lastly, they consist 

 nearly entirely of mesodermic tissues, the ectodermic epithe- 

 lium being reduced to a thin layer of cells fused with them. 



It is probable that the transformations of the splanchnic 

 laminae in the oesophageal tube may correspond in some 

 way with what Semper interprets in the development of 

 the head intestine in Nais and Ckatogaster as the forma- 

 tion of true branchial slits, homologous with those of Ver- 

 tebrates, which then become converted into part of the 

 oesophageal walls. 1 



Of canals and external orifices I have found no sign 

 in Lumhricus, and I have found nothing resembling the 

 branchial apparatus of Semper, unless it is the above-men- 

 tioned passage of a part of the splanchnic lamina of the 

 cephalic germinal streak into the walls of the head in- 

 testine. 



During the time of greatest activity of the mesoderm, until 

 a considerable number of segments are formed, the other two 



' Loc. cit. 



